Ok San Juan is a difficult place and believe me you have to be careful where to stop, not all the places are safe to do so. If you stay on the places near the principal road well you should not have any problem.
Usually on San Juan there is a lot of traffic so that will depends on the time of the day. Renting car on the island is easy, as mike the kid tells you, just after arriving there are a few rental places outside. As for driving on Puerto Rico, San Juan is not one of my favorite places to drive the people can go crazy depending on the hour of the day. Also most of the signs on San Juan now are changing to english insted of spanish for the tourists son in the future you will not have any trouble finding places on San Juan but on the rest of the island they will be on spanish not much to do there...
There are very nice places outside of San Juan and interesting places like:
* Arecibo radio radar station (1:30 hr. from San Juan)
*Porta Coeli on San German: A church an a similar town like old San Juan (3 hr from San Juan)
* "Las Cascadas" Acuatic park (2 hr from San Juan)
If you are looking for pubs and bars, well that will depend where in Puerto Rico you are, Here on the west side of the island there a lot of them and safer than the ones on San Juan problem is we are like 2 hour per travel from it.