In my uneducated opinion, it wouldn't be a bachelor or bachelorette party in the modern sense of the word - perhaps a banquet, a hunting trip with ladies tagging along with their fathers (in Nicholas's case) or perhaps a hosted opera or symphony concert.
Imagine (yes this is a horrible example) one of those "grand balls" on an old Disney movie where the parents are trying to marry off their children, and in turn lots of lesser aristocrats get to try for an heir on the throne. I mean, not on that kind of scale by any means, but does that give you any idea?