I'm going to have several extra days in Prague in May, and I'm looking for suggestions for day trips.
I'd like to see the Czech countryside, and I'd like to do some mild hiking - nothing serious, just enough to experience the outdoors. I won't have a car, so I'll be limited to where a train or bus can take me - although I could rent a car for a day.
Any suggestions.

Cesky Krumlov and Kutna Hora

Second vote for Cesky Krumlov and Kutna Hora. However, I will say that Cesky Krumlov has a romantic vibe and I would have enjoyed it more if I were not traveling solo. But then again, IMO Prague has a romantic vibe, too, so I just rolled with it. Some websites:

http://www.myczechrepublic.com/pragu...day_trips.html

http://www.praguenet.com/daytrips.html

Cesky Krumlov (I'd spend at least one night) and Terrezin

there are a lot marked hiking paths adroung Karlstiejn castle which is anluy about 30 minutes from Prague,you could spend a whole day in the area doing a visit to the castle and a little walking.
The Cesky raj area is a fantastic walking area to the North of the country but not really possible on a day trip by public transport.
When will you be there in May? I am visiting from the 13-20th may for the Czech Beer festival.
http://www.ceskypivnifestival.cz/en/
there are also some lovely parks in Prague and you can spend quite a bit of time wandering round them.

Thank you everyone. Wonderful suggestions. I already noticed the Czech beer festival, but I'll be missing it - I don't arrive in Prague until the 20th.

A good download of information. www.inyourpocket.com

Beer festival is on till the 28th ,bit I will be leaving on the 20th.