We like to plan fun days into our homeschool schedule. I've already talked about celebrations. Other fun occasions are park days, fieldtrips, and other days that you don't do school at home in the traditional manner.
If you have friends who are studying the same thing you are, plan a day together to do a lesson or activity. Once when we were studying a Shakespeare book, our friends' co-op was doing a play of that book! So we bought tickets and went to see it. We've done science units with friends. We've even helped Grandma set up her school class room, we got a little taste of what a class is like, met the faculty, and learned how to use all the teacher supplies (laminator, copier, huge electric staplers, etc.).
Park days can be a portion of the day that you spend at the park hanging out with other homeschoolers, or you can do a little work at the park then play.
We love taking fieldtrips. It's fun to coordinate them with what you are studying. The year we studied zoology, we got a membership to the local aquarium. Speaking of memberships and entrance fees, ask for them for Christmas! My sister and brother have given us tickets to the local zoo/aviary/aquarium and natural history museum as gifts for the past handful of years.
There are so many options for fun days that you can tie into learning. What are some other fun occasions you've had for school?