Always, The One to Rule Them All:
Shin Dong Wook in Soulmate (2006)
I had that expanding-accordion headband in high school 😜

wow! this is getting very serious.

I can only think of Ji Sung in Kill me Heal me. He wears a few bows/headbands while Oppa-ing as Yona.

Park Hyung-sik in Strong Woman Do Bong-soon immediately comes to mind. The leads were both so cute in those leopard/tiger headbands.

Basically any cutesy rom-com will probably manage to put the ML in one 😂. I think Business Proposal had Ahn Hyo-seop wearing one at an ice-rink/ winter amusement park?

Another memorable one was Kim Sung-cheol on his date with Song Joong-ki in Vincenzo.

Roowon in She Would Never Know/Sunbae Lipstick. Was FL applying skincare on him or makeup? Can't remember but he was cute.

Song Joon-ki's Saya getting a complete makeover done and therefore cameoing as a mysterious high-class woman on the streets in Arthdal Chronicles.

This one is not a cutesy headband but the head is involved anyway. Marriage Contract. How many times did Shin Rin-ah put a twig/branch of flowers behind Lee Seo-jin's ears?

okay technically it's a bandana not a headband but the one that YSY's character wore in The Best Hit before he got a 00's makeover later on (a shame honestly, that 90's look was 🔥)

In History of a Salaryman, Yoo-bang wears a headband when joining the striking workers. I can see it in the thumbnail for episode 11 on Kocowa.

Fiery Priest ep 36 - Hae-Il and Sun-kyu on a brotherly date. They wore headbands in a photobooth.

The cutest one that pops into my mind is Park Hyung Sik's twinning headband with Park Bo Young in Strong Woman.

However, for me the word headband on men paints images of these gorgeous Kings and Princes in Sageuks sporting these headbands that go with their warrior costumes. The guys are already so good looking, and these bandana style headbands coupled with the royal costumes make them look ethereal. I could look at them all day long.

Not a K-drama, but the legendary headbands of Daoming Si (Jerry Yan) from the OG Meteor Garden in the 2001 Taiwanese adaptation of Boys Over Flowers. They were hideous, but it was the 2000s and gave him his iconic pineapple hair.

Nam Joo-hyuk in WLF Kim Bok-joo matching hairbands with Fl

Junho wears a cutesy animal-ear headband at an amusement park in Just Between Lovers. Between that, his shaggy auburn perm, and the cuts and scratches all over his face, it's quite the look for an ML.

Lee Jong-suk wears one - with a red bow? - in the W - Two Worlds ice cream date dream scene.

Crossdresser Samantha (Ahn Woo-yeon) wears a sparkly jeweled headband in episode 6 of Mad for Each Other. I loved how accepting all the characters were of him including the apartment ajummas.

In episode 17 of Good Doctor, Yoon-Seo takes Shi-On out to dinner, on the pretense that she won a free meal. Cut to the restaurant, where the two of them are wearing these ridiculous headbands topped with giant bows. When the restaurant patrons and staff urge them to kiss, Yoon-Seo swears she had absolutely no idea that the prize was a couple’s dinner. Riiiiight….

I'll have to rewatch to confirm that So Ji Sub has a tiara when he babysit the kids in Terrius behind me.

When the couple goes to an amusement park :
- Strong Woman Do Bong-soon
- Just Between Lovers
- Weightlifting Fairy Kim Bok Joo
- Business Proposal
- Black
- Vincenzo (KSC)

In episode 3 of Yumi’s Cells, Yumi and Woong wear those silly frog headbands at that festival…and both in their own ways are none too pleased about it 😛.

When Park Hyungsik goes to an amusement park:
- High Society with Lim Jiyeon
- Strong Woman Do Bong-soon with Park Boyoung

Other ones that come to mind are:
- Kill Me Heal Me
- Weightlifting Fairy Kim Bok-joo

Kwak Si Yang in Oh My Ghost (2015) -- I remember thinking how surprising that something so naff actually looked cool on him.

Faith, Lee Min Ho as Choi Young. a character that still gives me butterflies. also his first love as a ninja also wore a headband, so do some of the Woodalchi