I agree with the cave idea. I have the neatest little cave for my betta, and he loves it. Sometimes I catch him simming in and out of it.
Another idea is to train your betta. That's what I've been doing. You'll be surprised how quickly bettas learn when food is involved. I've been training him to come up to my finger whenever I press it against the glass. If he comes up to my finger, I give him some food. He's getting pretty good at it. After he really gets it down, I'm going to try to teach him to follow my finger.
Challenging a betta like this stimulates thier mind. Its a good change from the monotony of a tank, where stimulation isn't as present as it would be in the wild.
Live food is another option. Brine shrimp (aka sea monkeys) will give your betta something to chase, like he would in the wild.