This was my answer to a birthday for a three-year-old who found soft play overwhelming. No games with winners, no loud music, no queue for anything.
Asking every guest to bring their own bear did something lovely - they arrived already holding something familiar, and the shy ones had a friend from the second they walked in.
The bear hunt around the garden took twenty minutes and was the entire entertainment budget. Ten small bears hidden in flower pots and a basket each to collect them in.
Tip
Keep it to ninety minutes and hold it in the morning, before the nap window closes. Hide the bears low - anything above a three-year-old's eye line simply does not exist. And have three spare bears in a cupboard for the guest who forgets.