Hand walking or grazing, giving them a bath, practising your braiding skills on their tail and mane, teaching them tricks, free lunging is always good and it’s fun when you include jumps.
Ground work, hand grazing, giving them a bath or a good grooming, walking then on the ground instead of under saddle
I take my mate for walks like a dog just for fun it’s good for introducing new environments and things to and give your horse better ground manners and they trust you more
Free lunging, playing around the arena.
There are lots of things, but that is what i do with the mare I ride.
Hope you play a bit with your friend!