There is only one option if you want to go here by public transport, but it is very easy and inexpensive.
First we took a bus from san Jose to Alajuela (1/2 hour).
From here we caught the bus to Poa (leaving at 9.00 am, cost $US 4 in year 2000). The ride took around 1 hour and was beautiful as we passed loads of coffee fields. Even though it was a public bus, it was clearly that nearly only tourists used it, as the bus stopped at some houses, where local women sold homemade cheese and other products.
The bus left again for Alajuela 4 hours later, which was plenty time for viewing the two craters. This may have changed since, though, as we visited in year 2000.
While you can drive, I would suggest using one of the tour buses. Of course I make this suggestion with most any place in this country.
The tour bus offers several advantages, first it’s comfortable, the drivers will give you a ton of information while traveling, and most of all… since you aren’t having to watch the road you can enjoy the county side.