Coupled with the right amount of planning, your job should be easy. It's easy to be creative for your kids' birthday parties. Plan toddler parties in the morning, as they get restless later on as the day progresses. Arrange for 20-minute games to keep the kids active and involved. Keep a list of extra games in mind just in case they decide they aren't going to play one of the original ones.
Free playtime is definitely an activity.
Consider having the free time outside, where there's lots of room for kids to run around and expend energy, thereby preventing chaos. Hand out small prizes after each game. They could be stickers, chocolates, mini books, fancy stationery items, etc. Serve a substantial snack or meal midway through the party to help refuel the guests for the rest of the day. Make each birthday party a dream come true.