The monday after Easter is a national holiday. In major tourist centres like Florence there will be plenty open ( we were in Florence this past Easter Monday and found plenty to do and see. ) In smaller towns your options may be more limited. I guess it can't hurt to stop in Orvieto and see what you can find. But come prepared with some drinks and snacks that you pick up in Florence before you leave there, just in case you might have to be satisfied with a picnic and a view! (So sad....) ;-)

By the way - since you're driving, be sure your tank is filled the Saturday before. I don't know if you'll find many petrol stations open on either the Sunday or the Monday, so better not take a chance.

Ciao! LL