Dividing the tank is a valid option, but I don't think Discus would be the best idea for your tank. They like very specialised water conditions and would probably be too big for your tank. Most puffer fish prefer brackish (slightly salty) water which the Betta won't like so it would be worth checking this too.
If you do divide the tank they can look quite good with a Male Betta on one side and a group of Female Betta on the other.
If you go for a community based on your tank size you could keep a group of around 15 Harlequin Rasbora in your tank, or a group of about 7/8 Harlequins and a similar number of Corys or Ottos. Corys prefer sand over gravel.