Oh my gosh, I dealt with this last July when my boys (13 and 5) were visiting their grandparents. A whole month off. It was truly a weird feeling, like, downtime, only washing my own laundry, only cooking for myself, what is this?
What I did- tried new things. Every day in the paper there's a list of free or low cost activities around town. I tried a free Reiki session, went to the Roller Derby, took long walks, browsed sidewalk sales, caught up on my reading with a stack of books from the library, sat down and watched movies on Netflix uninterrupted (!!!) without having to stay up all night. And Alanon, hot baths, all that other good stuff I usually do for self care as well.
Of course I missed them, but we talked every Sunday. The trick is, don't feel guilty. Taking time to recharge makes you a better mom, which is good for your kids. Relax, enjoy yourself. You deserve a little break.