Spring Break - Las Vegas

When we get a break from school, we have to get away ("we" excludes dear mother who will always deny wanting to travel). And after considering our options (Rachel always wants the beach and I always want Disney), the decision was unanimous (once again excluding dear mother) - Las Vegas. And it was booked. Seven nights at Bellagio. Roundtrip on Airtran from Atlanta to Las Vegas.

My sister and I have now been to Las Vegas three times (once in spring when we were seven and nine years old, once in spring when we were thirteen and fifteen years old, and then in spring break 2009 - fifteen and seventeen years old). Upon sharing our spring break plans, we've received the following questions.

Why Vegas?
What's in Vegas?
What can you (under 18) do in Vegas?

I hope I can answer these questions in detail throughout my trip report. I plan to follow my TR with my ratings, reviews, tips, and suggestions.