Celebration is a neat little area. Kind of looks like New England took over a small patch of Florida.
As for tennis watching, I think your best option is to watch a Power Player versus Bad_Call match.
You may just want to see if there are any Open tournaments at that time because UCF is a long way from there.
The outlet stores in Orlando are pretty good. I don't use Nike stuff so I can't answer about that particular store.
As for transportation to Legoland, that is a pretty good haul from where you are staying. If you don't have a rental car, I'm guessing a cab is your only option but that will cost an arm and a a leg.