That’s a great gift, happy birthday when it comes.
Yes you can buy individual tickets to yacht parties (apart from those which are invite only of course).
There are also several other parties and nightclubs in some of the hotels (Jimmy’z, Amber Lounge, Billionaires Club, etc.).
There is a new party this year but we can’t give any details at the moment until the final contracts are agreed with the BIG acts that we hope to have appearing. Keep in touch and I’ll keep you up to date with developments on this if you like. We also have yacht parties so if you want prices/details email me directly as I don’t like posting commercial stuff here, it’s not why I joined the forum, I’m here to help other F1 fans.
There are also lots of bars and restaurants, the who;e place is electric over the weekend, especially after the race so you won’t be short of somewhere to go. In terms of the best time to leave the track…leave when you have had enough fun :). If you are getting the train back to Nice the last one from Monaco is around midnight, not sure exactly but thereabouts.
If I can help with anything else for you or anyone else let me know.
All the best