Floating betta log
is what people are referring to.. they love them. They love anything floating on the top to be honest. I used naturally dried/fallen oak leaves and they will build their nests under them.
Definitely a few more plants- silk is ideal for bettas.. a couple of the small spiky plants you have can easily tear their fins. The one in the middle of your picture is especially nasty at ripping fins. Have to think of their fins like wet tissue paper- easily ripped.
Your tetras will love more plants as well.. I would say a good 6 more tall-medium plants at least would make the tetras happy!