My son is almost 5 and all year long he has been talking about his 4th birthday, they really remember at this age. We had never taken him to a "big movie" so we went to Ybor with his grandmother and two uncles and left his little brother with a babysitter. We went to a kid-friendly movie, rode the trolley over to Hooters and had lunch and then rode the trolley back to Ybor and had icecream. It was really cheap and honestly, he LOVED it. He got a balloon at Hooters and a paper trolley on the trolley and we all made a really big deal about it being his day. You could even offer for him to bring a friend but I really enjoyed it just being family. The aquarium is also down there but we had passes so that wasn't anything new. It was extremely simple and here we are 9 months later and he still remembers it fondly and it wasn't overwhelming in any way.
There is also an old Train ride in Parrish (close to Bradenton) that isn't that expensive and they do theme rides which are different every month. (www.frrm.org)We did one that was a Hobo cook out and they also do a halloween one and a Christmas one. The kids (4 & 2) absolutely loved it and had a great time as did all of the adults. Not the typical birthday party (or cost) but something they will still love and remember for years to come. Good luck!