L.C.
The thing I did for my BFF when she had her child was go to the store and shop for food. I spent two days cooking and produced two weeks worth of food that I put up in her freezer with instructions for reheating. That way on really bad days, she didn't have to worry about feeding her family. It was already there. I can't tell you how many times she thanked me over the next month.
If you don't want to do it all you can organize her friends and family and see if they would be willing to donate a day, a chore, an hour of babysitting, coming over so she can take a nap...basically anything that you think she could use help with. Then put it all on a calendar for her and give it to her the day of the party.
As a new mom it wasn't things I needed so much as just help. Knowing someone had dinner covered, I could take my baby to his well child without bringing other kids, my mom was coming over to help with laundry, I was going to get a nap that day. Those things were priceless.
As for the little boy, books all about being a big brother would be good and allow something for him and mom to do before bedtime.
Hope this helps