Yeah, Diglett's Cave and Pokemon Tower look like the best places at that point of the game.
Later, I can recommend Cinnabar Mansion, the lower levels of Seafoam Islands (surprisingly easy to reach and exit) and of course Victory Road.
Apart from that, there's too many Pidgey and Rattata line members (who obviously don't exactly give good exp) around. Even in the patch of grass with Tangelas that becomes a grinding paradise in FRLG.