I hang a hand-held mirror on the outside of my males' tanks to flare at. But they also like it when I float things around in their tanks for them to chase around (old lids, ping pong balls etc.).
However, the best form of entertainment I have found so far is live food. I release a couple of mosquito larvae and other water bugs into the water every few nights and they have a blast hunting them down.
I recently purchased a tiny almost minnow-sized boy (he was supposed to be a female) and as soon as I had introduced him to his tank, I let go a few mosquito larvae. By the next morning he had coloured up beautifully and was contenting himself with flaring at his next door neighbour.
Small statue's or pictures next to the tank Drax likes to look at.
Plants are probably your best bet... or Caves... Drax loves to rummage around the leaves. Stones are good too... to hide behind and look at. ... Really if you clean it and it's pretty... your betta will like it to. Remember... don't EVER use soap when cleaning. Just hot water.
Marbles is enthralled by his airstone...stares and the column of bubbles for minutes at a time. Also if you put him in a room that has a lot of movement, he will likely enjoy watching people move about. Marbles is next to my bed on my desk, so he sees me doing homework or relaxing on my bed and is always checking it out
I moved one of my really neurotic bettas out from the office where no one really goes, to a shelf on the kitchen. He really loves watching everyone come and go and whenever he hears the powder room door open or shut he sits at that side of the tank waiting to see who it is. It's amazing the difference it has made in his personality. He is a lot less stressed, and spends most of his time at the front of his tank, greeting visitors, my mum, the dogs etc.
I of course get the most enthusiastic greeting of all as I am 'The bringer-of-bloodworms-and-other-yummy-treats'.
I think I have too MUCH stuff in Drax's tank you'll be watching him... and he'll be watching you... but then you'll see him (Almost like a hyper active 3yr old) just sorta scoot off then act as if you aren't even there and start doing his own little thing. I don't know what it is but he loves to rummage around the stones in the tank. Even if there's no food there he's always trying to get at something. The larger glass stones are peach and sorta see through and I can SEE there is nothing under it yet he tries so hard to either move them or get under them. Perhaps he's seeing things.