As long as you are offering a variety of toys and social time, you’re doing great
. Buns are crepuscular, so they aren’t too active during the day. You’re right that mental stimulation is important, but even just spending time with your bun by simply sitting with him and talking or petting him is good for him too. And sometimes buns are disinterested in a particular toy and then all of a sudden one day will play with it again. I agree with the others that a digging box sounds like a good idea since your bun likes to dig.
Luna loves the treat ball as well. Instead of treats, I just use her portion of pellets for the day and put those in the ball. It gives her more time with the treat ball than if I were to use a couple treats, and it also helps prevent her from inhaling all of her pellets at once. I bought her these stacking cups a while ago that she likes to knock over and toss around too (but if your bun likes to chew on stuff he’s not supposed to, just make sure to put them away when you aren’t in the room).
https://www.amazon.com/First-Years-Stack-Up-Cups/dp/B00005C5H4/ref=sr_1_2_a_it?ie=UTF8&qid=1536357022&sr=8-2&keywords=child+stacking+cups