Our house is tiny (just over 900 sq ft), and unless it is summer, we only have one friend over at a time. I tell the kids the rules when they first come to our house. No shoes on the carpet, food only in the kitchen, and my room is off limits. Also, since my kids share a room (boy 2 1/2 & girl 5 1/2), they have to give the ok to their sibling if they're going to play in their room. Other than that, I let them have fun.
You've gotten some fabulous suggestions so far. I always print off a bunch of free coloring pages from the internet when having play dates. We get out all our craft supplies, crayons, markers, glue-sticks, scissors, construction paper, glitter pens, foam shapes, yarn, etc. Then I tell them to make whatever they want. It is crazy the things they come up with. Let their imaginations go wild. I also periodically save a couple of diaper boxes and let them play with those too. Playdough & dress ups are fun too.
If it is warm enough, we'll play outside with chalk, sand, etc. One time, I had my daughter & her friend use a magnifying glass & observe as many things in nature that they could. Then, I gave them paper, crayons, pencils, etc. & had them draw everything that they saw. They had a blast!
As far as snacks or meals go, I would make arrangements ahead of time with the parents. If it is lunch time, I'd supply the sandwiches, and talk to the other moms and have them send something else for the kids to eat. Otherwise, I'd just have cups for water (different ones for each kid so you're not washing 30 cups), maybe some graham crackers or other crackers, maybe some cheese slices/chunks, and/or some fruit. You don't need to feed them unless they're hungry.
We also have a rule that EVERYONE helps clean up. My daughter learned the hard way that it is much easier & more fun when everyone helps.
Also, like another mom said, if anyone says anything bad about your house, then they're not friends & not worth your time. I would think that they'd be grateful that you've invited their child to play.
Have fun with it! Good luck!
Shellie