I'm loving this post, it's like an antidote to all the horrible things in the news.
You could have a Brucey night with old style tv gameshows. It sounds like you have a budget so you could get a comedian to run it and do play your cards right, you could grab any couples out of the 'audience' and get them to do a generation game version of a dance, wherl of fortune etc .
A Greek night- break plates and dance.
A glamour night- get them to dress up and have glamorous men and women serve drinks. Give them funny awards.
Go round the world in a week/month. Every so many days do a non alcoholic cocktail, an alcoholic cocktail/ drink, meal, decorations, films, get staff to dress up and the residents too, language lessons etc.
I saw a video where residents in a home played hungry hippos with laundry baskets and light weight footballs with people in wheelchairs. Looked like loads of fun. Not sure how politically correct but everyone seemed to have an amazing time.
Speed dating... why not?
Get them to start a business and decide how to spend any earnings. Get them involved with it with local press etc and push it at local fairs/ car boots/ second hand shows.
My ds volunteered as a teenager for doe talking to old people. I know a lot of people have mentioned babies and primary children but secondary children can be lots of fun and both the residents and yp would benefit in a similar way. Obviously work with the schools on who would be sent etc.
You could have youth language lessons- I bet this would be really amusing. Translate the colloquial language used today for the residents with some young people (a-levels/hnc/hnd/degree).
A hemp class - legal products but let them try and see.
A burlesque show - old people aren't dead and why not likewise Anne summers etc.
Do any of the residents have any interests in certain types of vehicles or any special ones? Could you organise a day where vintage cars/ motorbikes etc attend.
See if any local businesses want reviews by older people or photo ops (glam them up and print it out when online or published).