It all depends on your betta, so be prepared to either return or rehouse the new fish. Also be sure to do some research on whatever you think of getting before getting it. And most importantly Quarantine for at least 2 weeks
. Even if you think you're getting healthy fish they can be carrying nasty things that can infect your betta.
As mentioned above, Mollies can be bullies, they also prefer a higher PH than bettas and (if I remember correctly) brackish water.
Platies are supposedly calmer, but be sure to get a 2:1 ratio since males will chase females and can chase them too much if there are not other females to distract them. If you don't want fry, get only males. Even if you get only females they are often pregnant and like guppies can hold sperm for a long time and continuously give birth.
Guppies or Endler's Livebearers are another possiblity, I personally do not know much about either of them, but I do know that there is a myth that male bettas will attack colourful male guppies. This is false, and if the betta cannot be kept with male guppies, then its likely not going to accept most other tankmates anyways.
Pygmy Cories! Cories need a shoal of at least 6 fish to be comfortable, but this makes them relax and be playful! Cories though need a cycled tank and a sand bottom/area.
Snails. Ah the classic snail. If you don't want snails that will make more snails, you can do a Nerite, or maybe
a Mystery snail. Nerite eggs cannot hatch in freshwater, so there is no chance. But they can be a bit picky with food, it's best to have a decent amount of algae for them in the tank before getting one. They might also accept algae wafers and blanched cucumber/zucchini but some will eat left over fish food as well. Mystery snails are also quite popular, but I don't know much about them other than the suggested minimum is 10gs.
RussellTheShihTzu is kinda the go to guy for betta tank mate advice!
This all really depends on your tank. I'm guessing it's a standard 10g? Also what is your filter? You just have fake plants right? how densely "planted" would you say the tank is? A picture might help :)
is a good resource but be sure to take its suggestions with a grain of salt.
Bottom line is, if your betta doesn't react well to tankmates, its better to let them be the only fish in the tank because betta are truly solitary fish. They in no way need tankmates and some are just not meant to, tankmates are more for our enjoyment anyways.