An experience that he would enjoy: an overnight trip (with you) to a nice hotel out of town, and a day of sightseeing, or picnicking, or going to museums, or new (to him) restaurants.
What are his interests? There are usually fun activities related to whatever kinds of things people are interested in.
Tickets to a play, a special meal either at a restaurant, or made by you. A formal photo shoot with you, or with his immediate family, or his pets (then get it framed). Go camping or glamping. Go wine tasting.