Let us first go over impossible, almost always deadly betta tank mate ideas:
another betta=fish soup
shrimp=extra meal for Mr.Betta
=Mr.Betta's all-you-can-eat buffet
snail=Mr.Betta's newest victim to his bullying
Now for some possible tank mates for Mr.Betta:
no tank mate=happy, healthy, lively betta that will be very peaceful and not stressed. He will live happily, sometimes up to 6 years, and you will be the proud owner of a betta with very vibrant coloration, and healthy fins
I suppose that the snail could work. I usually recommend them to people when they want a tank mate for their betta, but truth is, having another fish in his tank will just freak Mr./Mrs. Betta out. They usually become stressed, and their coloration fades. Please, just let your betta be by himself. If you really want a mystery snail in with your betta, go ahead, but don't say I didn't warn you.
No, but seriously. You COULD keep a mystery snail in with a betta. Just monitor them together for twelve hours before leaving them alone to break up any fights. Plus, provide LOTS caves and short standing leafy plants (silk is best) for the snail. Also, provide some tall plants to keep your betta comfortable too!