Bake him a "cake"? Carrots,apples,grain, idk..just make the cake fun and yummy! (: Maybe take him and some of your daughter's friends and their horses swimming if there's a lake/pond nearby? It's SO fun and the horses would enjoy it! Just spend the day with him..he'd enjoy that!
Hope it turns out to be a great day! HAPPY 5TH BDAY!