When I was a kid, my family did a lot of open houses/large group gatherings and so we got pretty good at getting the house ready in a few hours.
Most of the things to do are pretty obvious - sweep, vacuum, mop the floors, basically clean the house it looks good. Put any loose items away, like toys or even just stuff that clutters like newspapers or magazines.
Have the table with food against a wall setup in a buffet style so people can just walk by and grab a little food. Have lots of chairs, and set them up in small groups of 4-6 chairs so that groups of people can sit and talk. If there are going to be kids, consider a kids area outside, upstairs or in the basement where they can play.
Also try to leave some open areas around the food where people can just stand and talk.