The soda's as a general rule will not take credit cards. CR has mandatory tax and tip that is included on restaurant bills that is about 25%. If a place does not take credit cards, they don't always charge this. If you want to eat local, let us know and we can give you a list of great places to try. We all have our favorites.
Where are you staying? You may find an advantage to booking everything through one agent. It sort of depends on who you are renting through. I have found Daystar to have really good rates on their trips. Other companies really charge a ton and I would try to book directly with the opperator if possible.
I would do MA on your own. Just drive down there and look for the guys in the bright yellow shirts to tell you where to park. Then hire a guide. We have had great luck with this. I always tell people to try and do MA on a Tues or Wed. The crowds are smaller (they are closed Monday).
For zip lining, I like the one run by Daystar across from the gas station. I find it more thrilling than Vista Los Suenos, but both are good.
Many good surf schools. If you are mid Jaco, I like Tiger Surf School and Jaco Surf School. If you are North then use Chuck's Wow. Many will take you to the South end to surf, but I don't know the better instructors down there. I will tell you this can be a lot of fun and then you can rent a board for about $10 a day after your lesson.
Driving is a great way to go. Zuma Car rental out of Jaco will actually pick you up and drop you off at the airport. It is a great alternative for a first trip. They are also well priced. I think the last time I checked for a week it was just under $300. Getting onto the toll road can be a challenge, but your rental car company should be able to give you good directions. Just be aware that you MUST take the insurance on car rentals in CR. It is required by the government. The only company I would not rent from is Payless. The rates look really low and they end up charging you more than double what they quote. If you are arriving on a Sunday, then definately rent from Zuma. The toll road is often shut down for one way traffic which means you have to go the old way to Jaco. Trust me, you don't want to try and figure that out on your own. Let the Zuma employee drive.
Bring your ATM card and don't expect to use your credit card like in the states. Cash is still king here and trying to cash a travellers check is painful. You could spend half your day in a line.
You will do find with little Spanish. Never have had an issue.