New York New York rollercoaster, stratosphere rollercoasters. Quad biking through grand canyon. Hoover dam. That's all I can think of for under 21.
Seriously, I think vegas if your under 21 is a bit of a waste. I went last year when I was 22 and 1 month, got asked for ID about 1/4 of the time while playing poker, and all the time while testing out blackjack. Probably get away with playing the slots OK and the cocktail waitresses might bring you drinks, depends where you are. Aladdin casino was apparently most lax for being under 21, but I think its planet holywood now and don't know if they've tightened up.
Hmm im not so sure... I went when I was 21 and didn't get id'd in any of the cardrooms i went too (binions/golden nugget/plaza/MGM and the bellagio) however when I tried to sit down at 3 card poker at the plaza i was instantly id'd. No harm in trying to sit down at a poker table rooks.
Ill be in vegas from 6th-15th april getting my wedding sorted out

Good luck with the wedding, I got married in gracelands last year - was superb, and all went really smoothly.