For JHI:
* If you liked ONE SPRING NIGHT, then I’d definitely recommend SOMETHING IN THE RAIN next. It’s also a PD Ahn drama, so the vibe is really similar (just cover your ears whenever the soundtrack kicks in!). JHI and Son Ye Jin (who plays the female lead) have exceptional chemistry in this romance.
* I also really liked him in A PIECE OF YOUR MIND, but have tissues ready. Since the drama is dealing with heavy topics like grief, you might want to be in the right mood before you start watching.
* For a quick fix, I’d really recommend to watch the movie TUNE IN FOR LOVE, which is a really nice melo-leaning romance with Kim Go Eun playing the female lead.

For JSM:
This might not really fit the “boy/girl next door romance” bill, but in case “costume fantasy” is your jam, JSM was a joy to watch in ALCHEMY OF SOULS. This drama has a nice ensemble cast in general!

I echo @sonai in recommending those 3 dramas for Jung Have In. A Piece of Your Mind still has quite a special place in my heart after all of these years.

As for Jung So Min, I'd recommend Because This is Our First Life -- a more slice-of-life drama, but so comforting and so relateable

I have maybe unfortunately only liked JHI in his two first roles: The Three Musketeers, and Bride of the Century.

Jung So Min on the other hand was completely underwhelming in her initial roles which were mostly the sweet innocent candy. Then she went on to play the hilariously wacky second lead in Big Man which was a terrible drama, but she completely won me over! 😄👌

JSM was great in Because this is my first life. You can check her movie Love reset - its good with HEA and drama Alchemy of souls P1.

For Jung Hae In, as @sonai I recommend the movie Tune in for love. I liked it very much. (But I can't recommend One spring Night. I finished it but I didn't like it).
And my favorite drama, where he is a secondary character but you won't forget him: Prison Playbook.

He was very nice as SML in While you were sleeping too.

I have only watched Jung So Min in Because this is our first life and Alchemy of Souls. I want to watch The smile in your eyes too.

So Min: The Sound of Your Heart, My Father is Strange and D-Day (tbh, I don't remember how good she was on D-Day, I just liked the show lol)

Hae Jin: A Piece of Your Mind.

Everyone talks about DP, so I guess that would be good too. But I haven't watched it yet.
He was cute in Blood and Bride of Your Century too, but he wasn't main character and watching those shows just for him... Not recommended. Bride was nice at the beginning tho, so maybe you would survive the drag the last few episodes are.

JSM was very winning in Father is Strange, my favorite family drama of all time (and possibly the only family drama I have ever finished??). Would highly recommend!

JHI was great in A Piece of Your Mind, one of my favorite dramas of all time. As others have said, he’s also very good in supporting roles in Prison Playbook and While You Were Sleeping.

I’m triangulating for dramas I liked them in and dramas I liked—I think JSM was good in Monthly Magazine Home but I ended up really disliking that drama!

If you start with Something in the rain and stopped at ep8, you’ll love Jung Haein (and bonus: you won’t hate anyone else).

For me, you can watch every drama with JHI. If you can handle violence, D.P. was really good and will show a different facette of him.

JSM was really good in The Smile has left your Eyes. But it's not an easy story.

Jung So Min was delightful in My Father is Strange, it's a long ensemble family drama and does take a few episodes to get into it's stride, but it is my favorite family kdrama, rivaled only by Life is beautiful (2010). Incidentally, both share the same absolutely wonderful lead parent couple, Kim Hae Sook and Kim Yeong cheol. Jung So Min is lovely in it but the oldest sister character (played by Lee Yuri) is one of my favorite female kdrama characters, and Lee Joon (So Min's lead male) is a wonderfully intriguing ml so it has a lot going for it.

So Min is a lot more central in Because this is my First Life and I thought she was wonderful in that too and the female friendships in that one was very cute.

Jung Hae in I enjoyed in both A Piece of your Mind and One Spring Night. The latter a bit more overall than the former as a full drama. The only character i couldn't stand in One spring night was the fl's, Han Ji Min, father and the fact that he never got real consequences for being a pos to his daughters soured a bit of my enjoyment, but i thought the lead couple was really nice and their dynamics felt real and grown up. It's an ahn pan seok drama so be prepared for overplayed english songs, but it was balanced out by realistic feeling situations and dialog and character interactions.

For Jung So Min, I liked her in Because This is My First Life and Alchemy of Souls S1.

For Jung Hae In, I liked him in his supporting role in Prison Playbook.

For Jung Hae-in, you can watch:
While you are sleeping, he is the second lead but he was stunning.
He was also the main lead in Snowdrop but this is such a heavy drama.
For Jung So-min
There is Alchemy of Souls, a fantasy drama but she only appeared in the first part.
Also, there is Monthly Magazine Home, a light romcom.

I like Jung So-min in everything I've seen. I think she's a very distinctive actor. I recommend checking out Bad Guy because that's where she got her start and the entire show is an entertaining train wreck, and Playful Kiss, if you can put up with the premise and bland co-star, is where she showed she can shine as a lead. I liked her character in Big Man (ignoring the Kang Ji-hwan issues). Without caveats, as has been mentioned by others she's great in Father Is Strange and Because This Is My First Life. I also thought she was good in The Smile Has Left Your Eyes and Monthly Home Magazine, although I didn't finish those shows.

My favorite Jung Hae-in role: Private Ahn Joon-ho in Netflix’s D.P. (2021). He was terrific in that role and his physicality i.e. fighting was outstanding. Lol somehow I don’t think you want to begin your Jung Hae-in education with that one.

JHI out of competition reality show recommendation: JUNG HAE IN’S TRAVEL LOG (2019/20). (Available as a playlist on KBS World-Youtube). Reality shows are really not my thing but this is one of my favorite JHI’s ‘shows’ dramas included. JHI’s first visit to NYC. After a couple of episodes 2 of his buddies show up (Lim Hyoun-soo and Eun Jong-geon) and it is the 3 guys exploring around NYC and eventually going up to Niagara Falls with some spectacular footage up there. I lived in NYC (2013-2021) and I think this was filmed during October 2019 so there was beautiful autumn weather (including a stunning and funny night visit atop the Empire State Building) and you get to see a vibrant NYC before the pandemic in 2020 made parts of Manhattan ghost towns. Highly enjoyable. (I think JHI was 30 when this show was filmed and he ordered a beer at a place and the waitress asked to see his ID to verify he was old enough to drink in NYC. The drinking age in NYC is 21. He was a good sport lol.)

* For Jung Hae-in, I could recommend Something In The Rain, but I won't, because I'm not that evil. I haven't seen him in anything else outside of One Spring Night, which I'm currently watching.

* As for Jung So-min, she's excellent in Alchemy of Souls S1, which is just a great show in general. Outside of that, I also loved her in D-Day, which I think is a rather underrated drama, and one I really loved for the most part, though I may be slightly biased due to my love for disaster dramas & movies in general.

DP was very good, especially the first series. And JHI was excellent in it. But it's harrowing and hard to watch.

I also echo Isa Is Always Time Traveling--Prison Playbook! Such a great ensemble cast--I can't even decide who my favorite character is. JHI is not the main character but his arc is great and really shows his range as an actor.