Check out the north shore area of Miami Beach (say 68th - 80th streets on Miami Beach). The beaches are just as good (if not better b/c less crowded) than SoBe and it used to be that boutique motels in the area were affordable places to crash your head for the night. North Shore Park bandshell had cool concerts and there were a lot of more reasonably priced, but still amazing restaurants in the area especially compared to SoBe. The Miami Beach bike shares go this far north if you want to rent some bikes while you're around to hit up places like the Bal Harbour shops or the nudie beach at Haulover (ya know, if that's your thing).
Another place that would be pretty different from SoBe is Oleta River State Park. Off of the 163rd st causeway, it's a state park, but you can rent cabins if you act far enough in advance. The state park has lots of hiking and small beaches among the mangroves, but it's also just a quick walk over the causeway to get to Sunny Isles for a more traditional beach experience.
Again, great South Florida info, Mrs. PoP.
I would agree to avoid staying in South Beach. Gone are the days where my grandparents lived there in the late 70s where they got mugged a couple of times and life was slow and cheap. Now, the mugging happens in a more sophisticated way with overpriced hotels and restaurants. But it is a sight to see.
Like Mrs. PoP says, the North Beach part of Miami Beach (not to be confused with a neighboring town called North Miami Beach - you don't want to stay there!) is the up and coming part of Miami Beach and you can probably find some good deals on Air BnB, VRBO, and such. It spans the range from about 62 St. to 88 St. After that is Surfside and Bal Harbor, which will probably be more pricey.
Once you find a good place, you can rent the Beach bikes which the stations go all the way from the southern tip to about 94 St. Also, there are two good bus routes that go along A1A - Route S and Route 120 MAX (this is the express bus and better). Those routes both go from Aventura down to South Beach along A1A, then go to downtown Miami. You won't need a car. You can easily make it from North Beach to South Beach on the bus in 15 minutes (or less?).
I wonder if the Aventura and Sunny Isles area would be cheaper. It's on the bus route, but too north for the bikes. Oleta also has kayak rentals which are nice. If you can time it with a full moon, they have full moon kayak trips which are fun.
Make sure to try all of the Latin American cuisine. Peruvian is the new hot cuisine. There's a great restaurant in downtown.
Take the bikes down to South Pointe Park in South Beach and then ride up the boardwalk between the hotels and the beach. Really active and lively and fancy and beautiful.
