Grazing is a nice relaxing activity, and horses really enjoy that. Just make sure you are doing it in a nice place.
I always enjoy a good photoshoot with my horse, which I actually do quite frequently. The memories made are awesome, and I love seeing the progression of my horse throughout the time I've known her.
I've heard of people who like to do clicker training, I don't know much about it but it's worth looking in to. And also little tricks and things, which I also haven't taught my horse.
Ground manners are a great thing to work on. If you have a ring, arena, or empty paddock, lead your horse around and practice stopping, backing up, moving away from pressure, etc.
Feed your horse treats (not too many) but she'll definitely appreciate it.
Grooming! Get those natural oils circulating, or whatever you'd call it...
In hand trail rides! Walk around the trail.