It's usually rough the first day back, and I plan something fun toward the end... kinda something to look forward to.
I also remind them, yes we've been off, but can't be off all the time. We talk about how lucky we are to be able to take that time off.
I also notice that if my kids don't really "feel" like doing school and want to do other stuff, I make a short list that they can check off themselves. They work pretty studiously if they can see their accomplishments.