A lot of the more popular touristy stuff doesn't usually open till around Memorial day weekend, like the Medora musical. Don't know what your route is but Bismarck has the Heritage center located on the state capitol grounds. There's a lot of different things still open that time of year, but not it depends what you are into, for instance New Salem has the worlds largest Holstein, you can see it from the interstate, same with Jamestown & their buffalo. I don't know if Custers house & the Fort Lincoln block houses are open that time of the year, but you could check the websites to find out. Same with Camp Hancock on Main ave in Bismarck. Washburn has the Lewis & Clark interpretive center that should be open then. Just really depends on what you are into doing.