My cousin is planning our family reunion--44 adults & 5 children--in Las Vegas in
Not sure what your definition of fun is but... Get a house with a pool, get some alcohol and food and let the good times roll. When you can't stand each other anymore, it's time to check out the Casinos. Take a helicopter tour of the city. Crash some conventions. Send the kids to indoor skydiving. Have Gramps pop some viagra and get him a call girl, send Grandma to Madame Tussauds to be part of the exhibit. In all seriousness, spend a few minutes with Google, break up into groups according to ability and interest and have fun. If you can't have fun in Vegas you can't have fun anywhere. It's all what you make of it, but a banquet hall??? Not so much....
What part of the city is your house you are renting in? That will narrow the choices.
One place I would look at is the Wolfgang Puck cafe at the Spring Preserve. It is off Valley View and Charleston (near downtown). Not only is there a place for lunch but there are great ehibits and walking trails if you are going to be there in the middle of summer.
If you don't need a full meal, how about the Fondue lounge at the Ethal M factory? It is in Henderson.
On the westside, you could probably take over the small restaurant called Gina's Bistro or if you want something more casual, how about the back room of Wild Bufflo Wings. They have like 12 different type of sauces and other foods also.
You might also get a lot of responses by posting your question on Tripadvisor.