No, there's a new toll road from near SJO to the road to MA, and that road has also been newly paved. No adventure, but nice scenery.
If you cc covers liability, you will only have to pay the addition CDW, which will add ~$10/day to your rental. Be sure you have the paperwork from your cc company (a downloadable letter, usually), and make sure your rental quote includes the correct price.
There are nice waterfall-horseback tours a half-hour or so away from MA, near Dominical. (Don Lulos and http://www.villascostarica.com/dominical/waterfall-tours to start).

I don't know if your schedule is set in stone, but with a week, I'd try to go to another area like Arenal. They have great rafting, ziplines, etc. there, and it's nice to see more than one area of the country since it's so diverse.

good info. thank you both...

MA has a public bus that runs along the main drag every 15 minutes. We used it several times. It cost next to nothing.

We went with a private driver for our long legs. We typically do our own driving, but decided that it would be more relaxing to let someone else navigate. Our driver, Wilson Rojas, was the best. He was knowledgeable and helpful. He offered all kinds of advice and recommendations, and made stops along the way to view animals and places of interest.

We do the same as MF - private driver between towns and local transportation in town. If you do tours, they'll pick you up and drop you back off. Taxis were pretty cheap in MA, and we were able to walk to a lot of restaurants as long as we were careful along the road.

With 8 days, we stayed at Arenal and Manuel Antonio and enjoyed our trip in 2010 immensely, (check out my trip report if you're interested http://www.fodors.com/community/mexico-central-america/babymoon-in-costa-rica.cfm)

I liked having a car and with a GPS we had no problems navigating - although we did have some problems with our rental experience. We were forced to take insurance.

Everyone has to have the CR government mandatory liability insurance.

You won't really need a car while you are in Manuel Antonio, you can walk, take the public bus which is only about 50 cents a person, or take taxis everywhere.

I recommend these companies for some fun activities!

For white water rafting:
Amigos Del Rio - The guides are great, bilingual, make the trip tons of fun and safe. This time of year they run the Savegre River. Class II and III rapids, you also get to stop at a waterfall half way through the trip which just adds to the experience. They provide transportation and will pick you up from your hotel. It includes breakfast and lunch. Price $89 per person. Highly recommend!

For ziplining:
Same company, Amigos Del Rio also has an Adventure Park 10 in 1 tour.
They take you to San Antonio de Damas, about 30 minutes outside of Quepos/Manuel Antonio into beautiful mountains. You have the option of riding in a hummer or going horseback riding, I've done both and they are both great - just depends on what experience you are looking for. There is hiking, a Tarzan swing, multiple ziplines, and rappels all surrounded by waterfalls and lush rainforest. Beautiful place! $130 per person and includes transporation, breakfast, lunch and dinner.

Canopy Safari - great zipline company in Quepos!
They offer 18 platforms, 10 ziplines, 2 rappel lines, 1 suspension bridge, 1 tarzan swing, a butterfly garden and serpentarium. The guides are a lot of fun!

Hope that helps!

wow great information. Thanks everyone! I will post a trip report after we get back! 3 more weeks!!