Any time spent with your horse is considered bonding and building trust as long as you aren't harming them.
So you could do
-groom or bathe your horse but not fast, take your time, take an hour even
-take them on a walk, just as if they were a dog, horse are very curious animals and live seeing new things especially when they are with their owner, even a walk around the barn is effective
-grab a lunch/snack/book and just sit by your horses stall while you eat or read. You just sitting there not doing anything shows that you aren't there to always work or to harm them. Obviously don't ignore them, pet him and talk to him.
These are some of the things I did with my horse when I fell off and broke my back, couldn't ride for 6 months and I did this so that I could spend time with him and keep our bond