Rome has been really hot, and I would love to get my kids to a lake or river near Orvieto to play inthe water. We leave for Orvieto (andour car rental) today. Sara
Lake Bolsena is about 20 km from Orvieto and lovely to visit.
http://bolsenalaketuscany.com/
And Lake Bolsena is a very, very large lake.
You should really like Orvieto. It's a favorite stop for many travelers.