I would personally suggest taking karate, boxing, or kick boxing. The reason I didn't say Tae Kwon Do is this, I have seen many Tae Know Do fights, both on youtube and watching demonstrations (my sis took Tae Kwon Do) and there is a real lack of punches in that martial art. I mean they teach punches, but the likelihood of a I suppose the reasoning behind that is this, in Tae Kwon Do, you want punches to be your last resort, you will use kicks to distance yourself from opponents. I'm not saying it is ineffectual, but it is very one sided technique wise.
Karate is good, from what I've seen, it combines more punches and kicks than Tae Kwon Do does. However to be effective combat wise, you can't follow the techniques exactly as they are seen traditionally, you can't keep your hands down at your waist when executing Front punches and reverse punches. The kicks work as well as Tae Kwon Do and Kickboxing kicks, although I think kickboxing kicks are a bit more concise and sharp, therefore more powerful. Karate uses elbow strikes, but they are only effective in Inside Fighting. You learn a lot of kata (or ritualized routines) to memorize fighting patterns, which don't work too well in an actual fight. But Karate, overall is a good sport.
Now I must say that your best bet is kickboxing or boxing, boxing moreso if you're more interested in throwing punches rather than kicks. I am a former Golden Gloves boxer and a boxing instructor, and boxing punches are the easiest to learn, and are the most effect during a fight. The Jab, Cross, Hooks, Uppercuts, Overhands, they aren't hard to throw once you know how to use them and they DO land. If you want to sharpen your arsenal and throwing punches and kicks, do kickboxing. The kicks in Tae Kwon Do, Karate, and Kickboxing are very similiar, if not exact. But in kickboxing, you do quick snap leg kicks which can wear an opponent down.
So I will show each style and let you decide based on what you see.
Tae Kwon Do Sparring
[youtube]http://www.youtube.com/watch?v=9VopHPJqiKc&feature=related[/youtube]
Karate Kumite (note how many more punches than Tae Kwon Do)
[youtube]http://www.youtube.com/watch?v=W7cHghqmUEg[/youtube]
Boxing Sparring (light sparring)
[youtube]http://www.youtube.com/watch?v=SLZaRoJgjt8[/youtube]
Kickboxing Fight
[youtube]http://www.youtube.com/watch?v=mxMzYedmdN0[/youtube]