I am your expert! I go to South Beach about 6 times per year.
First, airfare. Have you priced out fares into Ft. Lauderdale? It's almost always cheaper.
Another thing to keep in mind. A cab from the Miami airport to South Beach will run you about $40 after tip, each way. There are shuttles (GO AIRPORT SHUTTLE) that take you from Ft. Lauderdale's airport to Miami Beach. For solo travelers, it's about half-price. You should ask about a group rate if you are traveling with others.
As for hotels, I have had luck with sites like Hotwire and Priceline, but be very specific about your location. If you want to be in South Beach, look at a map before booking on those sites. You don't want to end up "out of the way" if that's important to you. Also, I've had a lot of friends who had good luck with AirBNB. Miami has a lot of empty condos that need to be filled. Plus, your trip is the off-season down there. You can get a good rate. I have stayed in July for about $150 at The Mondrian, one of the nicest hotels. I got that on Priceline.
As for other activities, lay on the beach rather than buying chairs and an umbrella. Buy your sunscreen before you go. If you drink, buy a bottle of booze and drink 1 or 2 in your hotel room before you go out for the night. Be careful about "specials" at restaurants. Many are overpriced. Dine on Lincoln Road, not Ocean Drive.
Enjoy your trip. Miami Beach is always a blast!