Snails make great tankmates with a betta because of its hard shell which protects it from curious bettas maybe even hungry bettas. Apple snails come in a variety of colors and can get up the size of a softball. For this reason, you could need more than a 2.5 gallon tank to house a betta with an apples snail. Another type of snail that fits well with a betta is a nerite snail. These snail only get up to around an inch in length and are excellent scavengers. They are also great algae eaters and have interesting shell patterns. However, all snails are highly sensitive to copper so any medicine containing copper cannot be used in a tank with snails (or any invertebrae for that matter). A better choice would be to have a hospital tank and put the sick betta there to treat and leave the snails in the main tank.
Ghost/Red Cherry shrimp
Although most ghost shrimp are sold as live food for larger fish such as chiclids, they make great pets and tankmates. Additionally, because their bodies are entirley clear, bettas have a hard time seeing them so they cause any harm. Red cherry shrimp, my favorite invertebrae, is also great tankmate for many reasons. Because they only get up to an inch long, they produce very little to no waste meaning you can have 10 shrimp per gallon. Additionally, they are easily bred so that 10 will become 100 in a month or two. You can keep 10 and then sell the rest for a huge profit. Not only that, but they are one of the best algae eaters, behind the amano shrimp. If you had a choice between ghost shrimp and red cherry shrimp, definitely shoose the red cherry shrimp. However, make sure that there is plenty of cover and that the betta is well fed or else your shrimp may become an expensive appetizer.
Cories (Corydoras Catfish)
Cories (panda, dwarf, albino, etc) are another great choice for a betta community tank. Because cories are bottom feeders while bettas swim at the top levels of the tank, there will rarely be any confrontations regarding territory. Additionally, cories are a peaceful bottom feeder, unlike chinese algae eaters which don't even eat algae but instead on the slime coat of its tankmates. However, cories do best in groups of 4 more so at least 10 gallons is necessary. But with a group, you can enjoy the interactions of a schooling group of fish.
Similar to cories, loaches are bottom feeders and basically look like a bigger version of cories. Thus, a larger tank is necessary. In fact some loaches can get as big as 16" (clown loach). However, all types of loaches are peaceful. I recommend khuli loaches(4"), dwarf loaches(2.5"), hillsteam loaches(3"), and zebra loaches(4").
Otocinclus Catfish, or otos for short, have similar needs as cories but are much more harder to acclimate since mainly because most of them are caught from the wild and have not been bred in an aquarium environment. This causes them to be sensitive to any change in water conditions. However, once your oto survive the first 1-2 weeks, he'll live for a long time provided that the water remains stable and clean.
White Cloud Mountain Minnows
White clouds are similar to neon tetras in size and coloration but are more peaceful, hardy, and enjoy cooler water. Their temperature range is between 68-78 degrees but can survive in the 50 degree range. Their hardiness makes them an excellent fish for a beginner and their temperment makes them a great tankmate for a betta. However, the temperature of the aquarium would need to be in the upper 70's (78F) to accomodate the bettas needs and the white cloud's needs. Additionally, similar to the red cherry shrimp, white clouds are easy to breed and are believed to not eat their young (the betta might though). White clouds are a very good choice for a tankmate and for a beginning aquarist.