When Jean-Cloud said he was going to be a tennis racket for halloween, we thought he meant a tennis player, but sure enough, he dressed as a racket. If that's your bag, you'll need a large sheet of cardboard, and X-acto knife, some string, cloth duck tape for the handle and some fluorescent tempera paint. You might also want a co-operative dog willing to dress as a tennis ball to accompany you, but they're trickier to come by.
If you'd rather be a tennis player you just need some white clothing, a pair of sneakers and some terry bands. If you cannot find the right stripes for your whites, draw them yourself with a sharpie (on the socks, too). It's much easier than you would think. If it's chilly where you live, a white V-neck sweater is always appropriate on the courts. Pictured here: a little inspiration from Wes Anderson.
Pro tips: mind your footwork on dark streets, use a powerful forehand to extend your treat bag, and please, try not to make too much of a racket out there.