Around 16th Ave is usually pretty crowded on the beach. Around 5th Ave too. If you want to have a little more room go up towards the Bellair Beach end (24th-27th Ave). You can take a walk up through Bellair Beach and see some pretty cool luxury beach houses. 18 On The Rocks is a bar/restaurant on 24th with open air bar and fantastic food. Another place with great food is Keegan's on 15th. There's not much of an atmosphere there as it is just in a strip mall with no real bar, but the food is great. Try the Buffalo Grouper sandwich. For atmosphere, the best place is probably Crabby Bills on 4th Ave. They have a nice outdoor bar and open area, usually with live music playing. The food is solid, but not as good as Keegan's or 18 On.
Oh and if you go to 18 On, I know it sounds crazy at the beach in 80+ degree weather, but try the clam chowder. It is the best I've ever had anywhere.