Is one day enough for Niagara Falls?

Yes, one day is enough to see the main attractions, but you'll be rushing. If you just want the core experience—the Falls themselves, the Maid of the Mist boat ride, and a quick walk on the promenade—you can knock that out in 4-5 hours. I'd do the Maid of the Mist first thing (opens 9am, $28.25 per adult) to avoid the long lines, then hit the Cave of the Winds boardwalk ($24, includes a souvenir sandal) for getting soaked right at the Bridal Veil Falls. That's your bang-for-buck.

If you want to add the Journey Behind the Falls ($24) or the Niagara SkyWheel ($15), you can squeeze it in, but you'll be skipping any sit-down dinner or the nighttime illumination/fireworks (which run at 10pm in summer). Skip the Clifton Hill tourist strip unless you want overpriced mini-golf and a wax museum. Stick to the state park side. So yeah, one day works if you're efficient, but if you want to do the wineries or the Aquarium, you'll need two.