I reccomend, seeing a few historical sites during the day, relax by the pool or drive to the beach. The malls are also nice for people watching. My opinion is that Conde is okay to kill an hour or so, but it can get old very fast (for me at least). The website RonS mentioned has some nice sites and ideas too; but to be honest, nothing beats just relaxing at one of the many outdoor bars (away from car noise and fumes) with a beer and nice music playing in the background.
I'll be in SD with a buddy of mine around the same time as you (staying in the jaragua). If you want meet up with us for a beer, just PM me.
Cheers,
DM