we have 2 kids that have summer birthdays and 1 in the winter. We prefer the summer ones and have outside activities. The best ones are with water involved. We usually have each kid bring their own water guns, wear swimsuits and bring a towel. We set up a small pool in the middle of the yard which is the pool they will use to fill their water guns up and they play watergun wars. They really have fun squirting anyone that is trying to refill their water gun. That is like no mans land and when you need a refill you are a major target for everyone that still has water. The pool in the middle helps keep the kids from running in and out of the house to refill their water guns, it also keeps them from using the water hose to fill them. We would buy the cheap water guns I think there are 5 in a pack at the dollar store. The cheap ones were for any kids that forgot to bring their own.
Water balloon games are also fun. You can also do water relays where you have small bowls or cups and fill them with water, they have to balance them on their heads and walk across the yard to the bucket on the other side and dump their water in the bucket. The team that gets their bucket full first wins. Have a scavenger hunt-- get your neighbors involved and ask them if you can hide things in their yards for the hunt. A swimming party can also be fun. We have also had parties at the park and had over 40 people come as we invited the families so the parents and kids came. We would bbq hotdogs and hamburgers, have potato salad, baked beans, chips, watermelon, and cupcakes for the food. For games we did pin the tail and hung it on a tree, pinata, and they played on the playground. The park parties were easier as the playground keeps them entertained.
The indoor parties aren't as much fun and seems like you have to entertain them constantly to keep them busy and not trashing your house with going to every kids room and pulling out all the toys and running through the house everywhere. We play Disney Trivia, Cranium--pull out the easier cards that younger kids would know or get the child edition, charades, or other group type games. Twister is usually a hit and if you have more than 4 kids then you want to get more twister games and have them all playing it at the same time. Pictionary can be fun too. With big groups we just use the cards and make up 2 teams, then give points to the team that guesses the answer--more like win, lose, or draw which used to be a game show on tv.
Have fun and plan well. You need to have enough activities to keep the kids entertained for at least 1 1/2 hours. This can be a challenge. The presents and cake will only take about 1/2 hour and I usually do the cake right before opening presents at the end of the party. My sis-in-law had the kids sit at the table when they arrived and had a lot of art supplies out for them to make the birthday child a card. This kept the kids busy for at least 20 minutes and helped keep them in one area while all the kids were arriving. Some of the kids made more than one card for their parents or siblings so they took those home with them. You could also put out some coloring sheets that go with the party theme to keep them busy.