Last year, my buddy and I had a rental we used to get around town and to/from Jaco:
First, let me re-iterate what the guys have suggested.
Never leave anything of value in the car in plain sight. We were on our way back from Jaco and stopped at a roadside soda (small deli-type restaurant) for a something to drink. We had 2 surf boards, all our luggage, and some other stuff in plain sight inside. We were inside for less than a minute, literally. As we are walking out a young tourist couple (European) tells us that while we were inside for 45 seconds, a guy pulled up next to our car, got out, and checked to see if the doors were open. Mofo bus have been quick cause we didn't even see him pull up or drive away. Luckily, we locked the doors or we'd be without everything (clothes, money, passports, surfboards). Basically we'd be in deep shit. Be careful leaving anything in the car. If somebody breaks a window or jimmies the door to get in,
you pay for that damage 
. The rental insurance in CR is a bit different than here.
Second, driving in SJ is pretty crazy. I drive to work in Manhattan every day and even I was a little scared. Not of the cars, but of the reckless people WALKING. They either have a death wish or simply have no fear. But they will walk right out in front of a moving car. BE CAREFUL. We didn't want to hit anybody. The sidewalks are really narrow, so many people walk in the street, and the sides of most street have rain gutters. Be careful of them when driving and parking. If you get stuck in one of those, you ain't gettin out!
Also, like the others have suggested, it's pretty hard to know where you are most of the time. Most (90%) of the street in SJ are not marked.
PRINT OUT THE DIFFERENT MAPS ON HERE (ALL OF THEM). Each one has its advantages. We used them all since we couldn't find a good street map anywhere. All we had was the highway map we got at the rental place.