Welcome to TPT and a nice beta tank. That guy has better digs then I
You can do a ton of different plants in that tank. Which ones? It's up to your taste. You can check different plants in Plant Profiles
and/or other people's tanks to get some ideas on what to try first.
Fish wise, 6 gallon tank is not that big - that's good and bad news. IMHO, I would just keep the beta solo there but I know how you feel. Again, you do have a lot of choices and the selection partially depends on your personal likes. If you are looking at 'standard' fish ~1" in length, 4-5 Neon Tetras OR 3-4 Otos OR 2-4 Kuhli Loaches?
You are limited by the tank size first (can't put too many huge fish, not a good idea to add fish that needs a lot of swimming room like Zebra Danios) and then by the current occupant, your beta. Betas have different personalities: one might be just fine sharing the tank with 6 Endlers and another will chase them down, one will ignore guppies and another will try to shred them to pieces.
I would do some research on plants and fish and take it slow. You will have some hits and misses, but it's all part of the hobby.