I'm much older than you so I can't answer about places to go for spring break, but I can give you advice on the MB police. Don't mess with them. MB is a tourist town, but they take their rules very serious, especially during bike week, spring break, and other times when there is large crowds. The best piece of advice to anyone under 21 is don't get caught drinking alcohol down there. They will not cut you a break 99.9% of the time.