Something I like to give to Pickle is a loosely closed plastic easter egg with a treat in it. He really likes figuring out how to open it. Though, I do not give them to Pres anymore, because he just chews his way into them... Never have I seen him eat the plastic, but I won't be risking it! So probably not good for chronic chewers.
Pickle also really likes when I tie cloth around the bars of his cage. I still have not figured out how he's learned to UNTIE KNOTS... but he seems to enjoy it! It has taught me that hammocks are not his thing, and that he'd just prefer to untie them than sleep in them, LOL.
LOVE the Cheerio toy. Finally have something to do with all of these foam scraps laying around! I knew I hoarded things like this for a reason...