I biked in Portugal (during May, in fact) a few years back, starting in Lisbon and heading north. The Douro valley in northern Portugal was my favorite part. I have also heard good things about the Minho region in the far north, but we ran out of time and didn't make it there. Most of Portugal is hilly, but the roads are OK. Logistically, it was an easy place for touring. We returned to Lisbon from Porto by train without any problem bringing our bikes on the train.