For some people, getting a book completed and published is a one-and-done, and I can understand that. No matter how much of a labor of love it is, the whole project can be exhausting. Nevertheless, unless you've closed the door on writing further books, you don't want to get out of the habits of writing and of exploring new ideas. You do need space for rest and relaxation, but you don't want to slide over into the mental equivalent of twiddling your thumbs while waiting for something external to happen or for some fresh blaze of inspiration to strike.
Fortunately, you have a lot of options regarding writing, and some may even lead to new areas worth further exploration. If you don't want to commit to getting started on another book, consider blogging, journaling, writing articles, or developing content for a website. If you're a nonfiction writer, this might be a great time to try your hand at writing fiction or poetry---not necessarily for publication, but for the exercise of continuing to develop your writing skills while trying to stretch yourself a bit. (Poetry, by the way---at least good poetry---is a magnificent exercise in learning to pack the most meaning into as few words as possible, as well as developing your inner ear for the flow and cadence of your writing.) If fiction is your thing, perhaps researching and writing out a bit of history or tackling a different genre than your preferred type will keep the creative juices going and perhaps even throw fresh fuel on the fire. Breaks in your writing schedule are also good times for workshops or classes aimed at honing your skills. In addition, if advance readers or editors have pointed out problem areas with your writing---perhaps your grammar is a little weak or your imagery is too abstract---then a pause between major projects is an ideal time to work on your weaknesses.
The times in between major writing projects are also excellent times for catching up on your reading and for getting involved socially with other writers. If there's a literary group or writers' association in your area, consider joining in! If a local cafe hosts a monthly poetry slam, why not go? There are also opportunities in mentoring kids and young want-to-be writers, not least the fact that having to explain something to someone else on their level reveals how solid your own grasp is on the matter in question in a hurry. The important thing isn't precisely what you do when you've got what used to be your writing time on your hands; the important thing is that you don't let yourself stagnate.
Happy writing!