My suggestion is stay in your room! Miami is NOT the safest place to be if you don't know the area, especially at night. Bayside is nice but I'm not sure what time they are open until. By the time you get to your hotel it will be pretty late. I've spent a lot of time in Miami because I used to live in the Keys and that was the closest place to go for "supplies" and civilization. The last time we cruised we stayed directly across from Bayside and we walked around there, but during the day. At night we locked ourselves in the room and ordered room service.
Hi! Do not stay in your room. That is just dumb. We were in Miami a day prior to sailing last May and hit South Beach in the day time and Bayside at night and even walked back to our hotel after that in the dark! It is fine to venture out, just be safe and be aware of your surroundings, which is the same rule that should apply no matter what city or state you are in...even your home city!
I agree with sherri1. Play it safe! I've been to Miami a number of times pre-cruise, and only venture out during the day. Once, we took a taxi to a restaurant in South Beach for dinner, and everyone looked fine to us, but we were warned both by the taxi driver and the restaurant waiter to be very careful.