Daughters first play date :) AHH!?

so my 3 1/2 year old daughter is having a friend come over for just 3 hours for a playdate on Friday. This is the first friend she's had come over that wasn't a cousin or a forced friend (a friend of mine or my husbands child) so this is pretty exciting for her. She has plenty of toys in her room that they can play with together. I am doing pizza for dinner but all of this is so weird for me just because it is not a cousin or mutual friend, it is an actual friend... a daycare friend. They sometimes get mad at eachother at daycare but call themselves best friends (this sounds so funny) anywho... should I let them completely entertain themselves and only get involved if asked? Of course I will do my 10 minute checks (or more often) and be listening to make sure they aren't having an argument... or should I come up with some type of activity for them to do together? We have about 3 playgrounds in our subdivision, but it is getting pretty chilly here. My daughter is oober excited about this and can't wait for Friday... want some other parents ideas please!!