I, too, seemed to have missed why you can't ride anymore. Is it something to do with your health?
As for suggestions for non-riding horsey activities, I would definately suggest looking into driving. I've never driven personally, but I heard it is definately a blast. I would also talk to CheyAut because I know she competes with her minis in driving shows.
I don't know if you enjoy teaching, but if you do what about giving lessons? I occasionally teach lessons to beginners, and I have to say it's quite fun to see the smiles on their faces.
If I think of anything else I'll make sure to let you know.