You could do one of the "farm tours" that takes you out and about at the big farms if that's your sort of thing. Are you looking for strictly horse related things or open to all things? Is there anything in particular you really enjoy or prefer to avoid? Will you have access to your own transportation or be "stranded" or reliant on others?
I'm open to all things. There really isn't anything I'd avoid other than extreme things like bungee-jumping. :P My family isn't horsey, so non-horse things are mostly what I'm looking for. We will have a rental car.