Read that Arenal is the volcano to see. Also read to stay away from Limon on the east coast. Haven't been there yet, next June for me.

(1) Which volcano is a must see?
Arenal is definitely a must. Additionally, if you stay at the Arenal Lodge that's overlooking the lake and if you can afford to stay at the suites up the hill (costly but well worth it), you can watch the volcano with its gorgeous outbursts of smoke and fire, as well as the red lava flowing at night (it is flowing all the time but you can see it well at night) - all this you can watch at all times from the porch, your bedroom and even while taking a shower. We stayed there (November, last year) and although it was $100 a night at the time, it was well worth every penny. Spectacular experience!

(2) Is it easy to rent a car and drive around the country?
It is easy to rent a car. It is not cheap and it is by no means easy to drive around the country because of the road conditions. We rented a car from National, a regular car which was a BIG mistake. Rent a 4-wheeler. The important thing is not the 4-wheel feature but rather to have a strong and toll (high from the ground) vehicle. Road conditions are awful - lots and lots of pot holes, some very big and very deep. There were many places where we needed to drive in a speed of not more than 10-15 mph because of the conditions. The car we rented cost us with all the insurance and the federally-required deductible of $800 (even if you want to pay to have a lower deductible, you can't). next time we go to Costa Rica, we will make sure to rent a high vehicle.

(3) Pacific or Caribean coast??? Or both???
We haven't been to the Caribena coast mainly because, based on what we read and heard from people, the Caribean coast is not as pretty (that's the info we got). The Pacific coast is SPECTACULAR!

My husband and I travelled all around Costa Rica, by car. It is a gorgeous country, the people are just fabulous, and we loved every minute of our trip in spite of the road conditions.