An open bar can get pricey really quickly! I think a cash bar is fine. It sounds like a lovely, modest event; if I were invited to an event like that, I would not expect an open bar. I would make non-alcoholic drinks available for free, however.

Music from your parent's playlist would be good. Find out what music they had at their wedding, and what their first dance song was. They should be encouraged to dance to that song, alone, just like at the wedding.

Definitely have photos displayed of their wedding.

A toast would be good, but no games in my opinion. Games force people into participating, when some would rather observe. I always feel games are awkward and forced in these sorts of gatherings (showers and parties).

I wouldn't get too structured, since alot of the guests will probably enjoy catching up with each other and relaxing. Having a buffet dinner is a great idea, or at least heavy hors d'oevres with plenty of tables and seats. Walking around trying to talk and eat and balance a glass is never a good idea.

The brunch is also a lovely idea. A 30-minute drive is nothing these days; people drive that far to go to the grocery store. The people who want to come to that will; the others won't. It might be very nice for your parents to have an opportunity to connect with some of the guests more than they could at the larger gathering.

It's very thoughtful of you (and your siblings?) to celebrate with your parents.