What should I do for my betta?

I've had my betta for about 18 months now. We've had different tanks but the one that he's doing very well in is a half gallon tank. He's a very happy and nonaggressive fish and the only time he gets grumpy is when we change his water. He will kind of sulk for about 10 minutes but after that he's back to his happy self. I'm worried that he gets lonely or bored and I thought about getting a girl betta to keep him company but after doing some research I'm a little hesitant about getting him another fish to keep him company. I wanted to know if I should try to get another fish (if I shoud, what kind), or get toys and caves and a bigger tank to entertain him because he might be set in his ways of being by himself. Thanks!