A possibility might be the Rhinecliff Hotel, right on the river. In June, the views of the river should be great. They have a party room on the second floor which has a view of the river. Just a few miles from Rhinebeck itself. Went to a party there a couple of years ago; can't remember exactly, but I think the food was quite good.
4 Grinnell, Rhinecliff, NY 12574