I've seen Ouran High School Host Club and Ai Ore. Thanks in advance :D
coffee prince and sungkyungkwan scandal .. in both dramas does the girl dress up as a guy .. they are korean dramas :) .. hope you enjoy them :D
Ti Amo Chocolate (Taiwan), You're Beautiful (Korea) & Ikemen Desu Ne (Japan), Hana Kimi (Japan 2x, Taiwan and Korea) :)
Sweet Sweet Bodyguard (taiwan--100eps), Wild Romance (korean) both dramas girl works as a bodyguard.
If you can tolerate a long family drama, then Unexpected You fits your bill. One of the sisters is a tomboy, but it's not a gender bender. I really disliked this drama, but many others loved it so give it a go & see if it's for you.
First two that came to mind (after some of the ones already mentioned) were Last Friends and A Well Grown Daughter.
Other than those, there's a few female leads that aren't exactly tomboys, they're just not as femenine as the rest like in Gu Family Book, Secret Garden, Protect The Boss and City Hall.