Sounds like you're feeding him all the right stuff. If you want snacks for him, you could feed him butter or wax worms every couple of weeks or so. Keep in mind axolotls need a diet that doesn't have a lot of fat in it, as too much fat can cause kidney problems. If you're feeling up for it, you could get some cherry or glass shrimp for your axolotl to chase around. You would need to quarantine the shrimp for a couple of weeks (I've heard 30 days from some and two weeks from others), but after that you would be fine to plop them into the tank.
As for happiness, it all depends on what your axolotl's personality is like. Some of them like to float or hang around at the surface of the water, so for that you could get some little shower caddy platforms with suction cups for them to sit on. Some of them like to play in hides with lots of holes (like my own!), so for that you could find a cave or a ceramic log with lots of holes. And there are still others that love hanging on plants and other "precarious" places (again, like mine), so for that you could get some fake (or real) plants for them to sit on. Or, and I thought this was genius, you could buy one of those parrot toys made of plastic rings. I bought a star-shaped one a couple weeks back and my axolotl just loves it!
Anyway, sorry for the paragraph.
But I hope I helped a bit!