In this case, Ultimate Frisbee actually means any kind of overly athletic theme. You may be an über athlete, but if you're going with some kind of theme that requires a lot of physical exertion, you'd better be sure everybody there is up for it. People generally want to go to a party to hang out with friends and relax, not beat each other in a potato sack race. So leave the hardcore sports for your leisure time. If you really want to do something active, keep it low-impact and fun, and set up a volleyball net for those who feel like getting sweaty.