Dancing competitions are fun. It doesn't take any structure and you can easily give everyone a prize. You just play music and everyone has to dance like a robot, then you give an award. Play music, everyone dance like the queen - give an award. Play music, everyone dance like a rockstar - stop music, give award and so on until everyone has a prize.
For kids under 10 I would say Pass the Parcel, Pin the tail (or you can tailor it to suit the party theme i.e Pin backpack on Dora), Egg & Spoon race and Statues (when the music stops you have to freeze). For older kids I would suggest a digital scavenger hunt where they are given a list of items to find and they must take a photo of them for proof. It's easy to organise, costs nothing and is really fun.