What is the best type of bird I could keep for a pet?

I have a bird cage, not big enough for a budgie or parakeet. It's more for the size of birds like finches or canaries. What types of small birds are out there on the pet market? I've looked into the different types of finches. Which ones do you recommend? And what about canaries? Are there any other kind? Can i mix different species? How many can I keep? I'd like some info on which ones are the hardiest or most active, etc... Thanks