frozen grapes are yummy.
If you get those bags of frozen fruit, you can pop them in a processor or use a stick blender and mix with some yoghurt. Instant ice cream!
I second the cooking with them.
I find getting them to grate veg is really good. Or cut up toms, peppers courgettes, cucumber. Popping peas from pods.
They also love the magic of using stick blender to make soup...making the food disappear.
Of course, none of this food is for them. Oh no. Its for you and daddy, or whoever.
All the time I ask them if they think we have made enough, are the pieces the right size...sort of gives them pride and ownership,
Another good way is to get them to make a tomato sauce base. This is messy but effective - big bowl, peeled canned toms (I tend to cut of the stalky ends as they can be tough) and get them to squish the toms right down. This can be used for a pasta sauce, pizza sauce (pizza base can be bread, pitta, whatever you have to hand).