Krista on stumptuous.com does BJJ - It's pretty kewl and physical and rolling around close contact wrapping your legs around people's heads
Personally I don't really like grappling *gulp* but I have to do it for my next belt test so I'd say suck it up!
It depends what kind of movement/technique you want to learn. Karate/kickboxing are more aerobic in that you probably burn more energy and promote more strength in your core and legs since you have to stay upright and kick and be balanced, whereas I think Ju Jitsu is a bit of an easy option since you get to lie down!
Go for a trial lesson and see what it's like!