Do not hesitate to tranq if needed - it will save BOTH of you a lot of stress. You can always decrease it later as the horse settles into their new routine.
Slow feeders. Lots of hay through tiny little holes. I have a portagrazer, and have the small-hole nibble nets. You can even net the hay then put it in the portagrazer.
Hand grazing as much as possible. I would go once a day in the mornings to hand graze, and hired someone to get her out in the evenings. I think that routine really helped her feel like she was still getting out. I also would put her in the cross ties as much as possible (e.g., when picking out her stall) just so she got as much time out of her stall as possible.
Clicker training for sure. Lots of silly tricks you can teach while on stall rest.
Learn to do some body work - Masterson method is a great place to start: lots of free videos online, super gentle, horses love it. I just signed up for Equine Bodywork Online (there’s a sale right now!). Not sure I’m 100% sold on it but it’s giving me lots of ideas. I’ve always considered bodywork something I pay someone else to do, but this has given me time and the opportunity to try a few things myself. Pony seems to be really enjoying it, and it’s been great for our bonding (I still have professionals come out to do the real work, I just try to fill in in between visits).
If allowed per vet, get some foam cushions (like the SureFoot pads) and play around with those (this might be something that waits until later in recovery). Some people buy cheaper human version online (I just ordered some from amazon - tbd how well they hold up). Similarly, standing core work, carrot stretches, etc to keep some core fitness up (again, if allowed per vet).
This is a great time to really get to know your horse’s body - where they hold tension, how they hold tension, and how to help them release it. My horse has been recovering from hindlimb PSD surgery the last few months and I’ve really enjoyed the journey in learning more about this (despite the frustration of the injury and all the stress associated with it). It’s been very interesting exploring different modalities, and getting to know her at a much more intimate level.
I know I’m going to emerge from this a better steward of her health and physical well-being. It sucks, and I’m sorry you have to go through this, but this is a great opportunity to slow down, and really “hear” your horse.