We just had a party with a similar age range and just letting them play with DS's toys was perfect. I don't think there would have been time for any kind of 'structured' activity, but it was a birthday party so we already had to fit in gifts, cake, etc. Plus none of our friends had ever visited our house, so everything was new to them and therefore pretty entertaining.

We don't even keep many toys out & I forgot to drag out a few from storage like I meant to, but the kids still seemed to have fun.
Several of the parties I went to recently had a cardboard structure -- train, playhouse, etc. for the kids to play in, and several of them had a simple craft -- one was decorating party hats with markers & stickers, one was a painting craft, another was decorating the cardboard playhouse... so those kinds of things were fun but I don't think "necessary" with such young kids! If they seem bored, you could always put on some music & have an impromptu dance party, or break out the playdough...