You can try reserving a smaller car than you need. Like a cheaper 2-door. Then when you get there, tell them it's not big enough. You can generally get upgraded for less then what the bigger car cost to reserve.
If you were staying longer I would recommend buying a car. Ricans have all kinds of old cars sitting around that run strong and are very cheap. We stayed for a couple of weeks in Ricon back in 2004 and bought a '73 Chevy Malibu. We bought it with a guy that was staying for a month and sometimes 7 of us would pile into this thing and cruz all over the place drinkin and surfing. It was known all over Rincon and everyone knew us by the end of our trip. The dude that stayed for a month ended up selling it to a bartender at the Calypso for $250 and we only bought it for $300. So he got a car for a month basically free. hahaha
check out charlie car rental. im not sure if they have an office out in rincon, but they have several around the island (im pretty sure theres one in Aguadilla and several in San Juan) and every time ive gone to PR it has been much cheaper than all the other places. you can book everyhing on there website:
Check out Target rental cars if you fly into San Juan... They were like 100$ less than other companies, and their fee for under 25 drivers wasnt as steep either. The airport might even have a shuttle to their lot which is like 2 miles away from the airport...
dont saty in Rincon! its a cool place and great surfing when there is a huge swell but you'll surf way more in Aguadilla. It's only a halfhour from rincon.
stay here- http://www.lukersurfpr.com/
awesome guy from jersey owns the place. he's got a wealth of local knowledge and he is always willing to share. a ton of boards too. i stayed there 2 years ago for 3 weeks. it will totally enhance your experience...even if you have reservations already, CANCEL THEM!
super cheap and a nice place with kitchen.
TOM IS THE MAN~!
and rental car cpmanies ALWAYS charge for UNDER 25. every place ive ever traveled. but i was always willing to pay it because until i was 20, you couldnt rent a car until you were 25 PERIOD
-shop around though because prices vary. local small businesses are usually cheaper than the big chainstores
and fly into Aguadilla
~hope this helps! and enjoy...PR is beautiful...bring your snorkel gear