Just to clarify... Bettas dont want/need company but they can live with tankmates.
But yes, one betta in a 10 gal might be plain. Your best bet for tankmates would actually be bottom dwellers. Bottom dwellers look/act nothing like bettas. Besides, they wont be constantly "in his face" so less chances of bringing out aggression. I would recommend corydoras catfish. There are many varieties available but one thing to take note of is that they need to be kept in groups of at least 3 of the same species (as cories DO like company).
One betta and 3-4 cories will be pretty much it fish wise. you could also add a few shrimp as well.
If you dont want to do cories or catfish, you can try harlequin rasboras, neon tetras, black neon tetras, etc, but chances of agression may be higher.
When you stock up the ten gallon add the betta LAST. If you add fish into a tank that has already been 'claimed' by a betta then he may feel a need to defend his territory. But if you add the betta last, he wont feel like the other fish are invading.