We're heading to Europe in May too and are staying two nights in Vernazza. On a previous trip to Europe, just did a day trip and were disappointed we didn't slot a few days for Cinque Terra.

For this trip, we're going to spend one day hiking between the towns and spend the other day taking it easy & checking out the shops to find some pesto and wine to bring back with us. I believe market day in Vernazza is on Tuesday.

Just a heads up that it might be difficult to find a place to eat during "non-regular hours" (ie. between lunch and dinner) as it seems a lot of places close during this time (at least in Vernazza). Plan your meals accordingly.

I read on some sites that people have driven to Cinque Terra as part of an overall driving trip. However, there really isn't a need for a car between the towns and I hear the roads are a bit narrow and windy. We're ditching our rental on the leg before Cinque Terra and training it there.