My trip reports might help you with some of your questions. But as a quick answer to "what not to miss", my thoughts are:

Arenal:
1. Cano Negro Tour (river boat ride along river between CR and Nicaragua where you see lots ot wild life).
2. Volcano Hike (you hike through national park to get close to volcano. You get to hear and see lava).
3. Hot Springs (many to choose from: Tabacon, Eco Thermales, Baldi).
4. Horseback Riding to La Fortuna Falls
5. Hanging Bridges/Canopy Tour

Monteverde:
1. Hike in the Monteverde Cloud Forest with a Guide
2. Hangin Bridges/Canopy Tour

We did the following from San Jose as a Day tour:
1. La Paz Waterfall Gardens
2. POAS Volcano
3. DOKA Coffee Plantation Tour

Can't comment on the driving but the roads from San Jose to Arenal are fine. In and out of Monteverde are pretty bad, dirt roads with potholes. Another option is to use interbus (can book online) to get around, which is what we did. They are on time and you get door-to-door service.

As for restaurants, I recommend the Tree House restaurant in Monteverde. In Arenal, I recomment Don Rufina, La Choza de Laurel Restaurant and Las Brasitas (Mexican). I have the links to these restaurants on my trip reports:

http://www.fodors.com/forums/pgMessages.jsp?fid=6&tid=35095425&numresponses=3&start=0&keyword=165

I live in Costa Rica and will answers some of your questions. Cel phones from other countries do not work in Costa Rica. You can rent cel phones from lots of places but be aware that prices for use are high here. You can buy phone cards here to use in the pay phones and from any other phone.

If you can afford to, rent a tall car, that ususally means a 4x4. The main reason is the roads can be pretty bad here and you need the ground clearance. Also, to get to where i live, it's 4x4 only!

Be aware that driving here in Costa Rica is like running with the bulls in spain. Many die, but the rest have one hell of an exciting time!

I have a web site about vacation travel in costa rica you can review. Maybe you'll learn something. For sure you will find some funny words.

http://costarica-pr.com

I have ATT/Cingular service and my blackberry didn't work in Costa Rica.

Driving in Costa Rica isn't bad, just drive carefully and avoid driving at night. Rent a 4x4. I regreted not renting a 4x4 while I was in Arenal & Manuel Antonio. Even though roads/highways to Arenal and Manuel Antonio are in good conditions once you left the main roads, roads are pretty bad. They are't paved w/ lots of potholes (i.e. roads to Arenal national park, 4 km on unpaved road to La Fortuna waterfall, etc). I agree w/ Mountain man that you need a 4x4 to get clearance.

We didn't buy the map, but use the free map I got from Peace Lodge. We got around fine w/o getting lost. If you aren't comfortable w/ map, you can rent a GPS.

I recommend Soda La Parada in La fortuna. We ate there twice and thought it was really good.

Enjoy your trip!

How many nights do you have? I'm a little concerned with your time frame, that you say you only have days. Also, you mention Ballena Park - this is near the Uvita area and quite a drive from Manuel Antonio, I'd say 3-4 hours (and the first hour of that to Dominical over very rough road). Also, Juan Castro Blanco Park is going to be roughly 2 hours drive from SJO and approx 1 hour from Arenal. So, I guess the most important thing would be to see how many nights you have available to see if you can squeeze all this one but it sounds like you will be driving a lot, may want to scale it back a bit.

Thank you to you all. You are very kind! In turn, I hope to look at everything a bit differently so that I may efficiently report back. And of course take lots of photos.

My family and I will be in CR 10 days.
Guess I screwed up on the car rental ...it will be a Corolla from Thifty.

Hope to get to Ballena Marine even if we have to go with a tour group.

Happy Holidays to you and yours.
Ribota