I do joint Birthday parties with my friends every year, we are about 20 girls, all with kids, and I do not want to go to 20 parties in a 3 month period!
We usually rent a local church hall, and use their playgroup toys. We had a party for Noah and his friend Harry about two weeks ago and we rented a bouncy castle, and for food all we did was Pizza, crisps, veggies & Hummus and cups of ice cream. It was easy! We usually get one big cake, but this year the boys wanted their own cakes, so Harry had Spongebob and Noah had Lightning McQueen. We DID NOT do party bags, as they are a palaver and IMO the kids didn't miss them as long as they got cake to take home.
If the weather is nice, I would have the party at a facility so you don't have to do any work. Unfortunately for me, I have a January baby and am now having a March baby!!! Never again!!! Good luck!!!