There are a lot of different things to do. You can just lie on the beach (don't forget to use sun-blocks) or spend your days on feet, exploring town's streets and the botanical gardens, or do shopping, or go and visit Barcelona and other towns on the sea coast. In the evening you can go clubbing or just walk along the beach, where you can find a lot of interesting things, including street artists and musicians.
Fondest memory: My first wonderful memory is the sea at night. I know, it's nothing special, but for me it was just great. The sound of waves and the lights of the town are a really good combination.
The second one is Mar i Murtra botanical garden. It is the "must see" place, really really beautiful.
I like the promenade at the old town end. There are pleasant gardens and as usual in Spain, small people are well catered for. Sit and read in the morning sun, with nice views of the ocean while your little ones let off steam on the swings and slides.
There are some nice restaurants down this end, near the port too.