Upon finishing a week of surf camp in Malpais, my friend and I will have 7 days to tour Costa Rica. We would like to visit Monteverde, maybe Arenal, and Manuel Antonio (I know, I know, it's too much). After reading a lot of your great posts, I am unsure about renting a car or taking buses, any advice? I also would like to know if it is possible to rent a car in the Malpais/Tambor area and return it in San Jose? Thanks!
Budget has a car rental facility near Frank's Place in Mal Pais. There should be no problem renting from there and then returning it to San Jose

if you can afford i would recommend a car. this way you can visit the places you have mentioned in a week easily. roads have improved a lot except the ones to and from monteverde. small terios is good enough. 4wd is necessary if you want to go to monteverde. malpais - arenal - manuel antonio will be okay even in an ordinary car. of course you can take the bus but be prepared for long hours and few connections. one way rentals should be available - probably even at no extra charge when you return the car in san jose or at the airport.

Consider taking the bus to Montezuma and surfing there for a afternoon and party that evening, then take water taxi to Jaco. Pick up car there for Monteverde, or take bus from jaco to Quepos, very easy straight shot, and then head to Manual Antonio.
But, I would even consider nature wise, forgetting MA, and try to get a tranfer/shuttle to Monteverde, then Jeep>Boat>Jeep to Arenal, and do a side trip from Arenal to the Rincon, and the Rio Negra and back to La Fortuna, then bus or transfer back to San Jose area.
Another option, is do all this first:
SJ to Arenal- 4 hours car/bus
Arenal to Monteverde 4 hours all modes of transport
Monteverde to Jaco 4 hours
Jaco to Montezuma 2 hours
Montezuma to Mal Pais 2 hours
Fly Back to SJ
You can take rental cars one way.

Try POAS . They, like Enterprise in the U.S., will come pick you up (but POAS leaves the car with you), and you can drop it off anywhere. But be double sure to check for scratches, dents, etc. BEFORE accepting the car.
Bus is a nice experience if you have the time, and can't afford a car.. Also consider a 4x4..
I know Ticotim thinks the Terios is junk, but I found it is a tough performer. Amazing is a better description.. A Terios is so easy to break into with a simple screw driver, that you should NEVER leave anything visible in one when you are away or sleeping.
You must pay for the repairs when it is vandalized.

Yea...I am not a big fan of the Terrios, its under powered and a very small car for the dangerous roads and mountains, trying to pass and also all the huge tractor trailers....prefer more metal and also more power, and safer height.
If you get all the insurance, you do not have to pay beans for anything, you can bang it up all you want, just get a police report, and report all damage soon as it happens, and dont move car. The only amendment to the insurance is if you are under the influence of drugs or booze, its all null and void, so beware...
Never, ever, never, ever, never, ever, leave anything in the car, not even sunglasses or a CD.