I also put crinkled paper in with the nuts so my bird has to work a little bit harder to get the goodies. It keeps him busy, plus I think it gives him satisfaction to make a mess out of the paper.
Try different kinds of nuts. Pistachio nuts are the best because they won't fall out of the holes so easily, which means the bird will work harder to get it.
You can put whole dried red chili peppers. Won't really spoil plus it is a good treat moderately high in Vitamin A.
Nutriberries would work too, make sur eyou buy the bigger kind.
I don't really put fresh fruits/veggies in the foragers because of spoilage. I don't even put dried fruits in them too because overtime it leaves sugars and sticky residue that might encourage bacterial growth.