Dario dario are another option. A single male would do well in a 2.5 gallon tank. I would not keep two males in a tank that size though as they are actually very territorial little fish.
Clown killifish would be suitable for a 2.5 gallon tank. I personally think most of the other species really do better in at least a 5 gallon tank as they are extremely active. My australe pair share a 7 gallon and they use every inch of it over the course of a day.
Sparkling gourami are another option. I had some and they are lovely fish.
Keep in mind however, most of these nano friendly fish do require good water quality. They are not as hardy as bettas and so less tolerant of poor water conditions.