For a first day, I'd groom a horse and then take him for a little walk, leading him, not riding, and dealing with any respect issues that might arise from ground. Also, petting him a lot, to explore, how high is his trust level and how comfortable he feels with me. Watching him in the pasture sound like a nice idea, too. The next few days I'd do some groundwork, and only then go up to riding. But I like to take things slow and make sure that the horse accepts me, before mounting. :)
I have come a long way, to surrender my shadow to the shadow of my horse.