Anything, really. I don't think it "is" a combo, as far as I know, the proper word for it is "string." A string is (again, I could be wrong) a bunch of moves that you string together effectively, that make a decent combo, but can be escaped. Your opponent can escape it with good DI, dodging, or counterattacking, but if they don't time it right, or you vary your timing so that you're unpredictable, you'll be able to land multiple hits.
You could like, upsmash->uair uair uair
uair from below a platform -> uair uair uair
My personal favorite though is uair->Bair. A good opponent won't normally fall for multiple uairs.