Last year I went all out, because I had a bunch of school aged kids. We had themed food all day, did crafts and games, and coloring sheets, etc. We also came in costume that day.
This year I will encourage everyone to come in costume, and we will color, have some fun snacks. But I am scaling it back a bit, since I will only have one (not quite 2y/o) kid that day, plus my daughter!