Hi
I as there at the end of last summer for 10 days and think that was about long enough to relax and get things done.
There are obviously the usual things to do, beach sports etc. As well as that we spent a day up the mountain, you can get a cable car up to almost the top of it and then there are lots of walks round the top of the mountain etc.
Go to Mijas for the day, it's a little unspoilt town with lots of shops, restaurants etc, and maybe do a day in Torremolinos (walking distance from Benalmadena).
As for eating, if you go down round the marina there are nice restaurants but they are quite expensive. Find Bonanza Square if you are after cheaper places to eat, all the restaurants there are similar and offer a starter, main course, dessert and bottle of wine for about the third of the price of the marina restaurants. It is a limited menu but because there are so many restaurants offering the same deal but different meal options, we were always able to find something to eat!
Google to see if there are any fairs (Ferias) on while you are there, there was one in Fuengirola while we were there and it was a different night out to the usual.