We know a little secret: if you don't want to pay $25 for parking, there is a free one here, in 15 min walk from the Horseshoe:
On a long weekend, if you like to take extra sleep and come here around noon, you may need to make 2-3 rounds to find a parking spot. We did 3 circles this time but found it. Nice picnic area, ponds, birds, - it's all there. Have something with you - coffee, fruits, energy bars, whatever. The place is very convenient for both a light lunch and full sized BBQ. There are picnic tables, bathroom, some trails and bridges. Do it all and then walk to the waterfalls (other way around may work too but we, for instance, are usually exhausted after the walk and simply drink, jump in a car and drive home).
Unless you are willing to pay $3.75 for a tiny soda, take some water with you, head to the river, then walk down the stream to your left for about 15-20 minutes and you cannot miss it. Remember: if you want to see the rainbow, come at a time when the sun is behind you.
Niagara Falls are extremely touristy, but weekends are especially crowded, so if you plan to have some extra fun like a boat tour
then you probably need to come early or on a working day. The lines to those attractions are long, but everyone who wants it always manage. Do your homework and look around: there are a lot of other things to see, like a Floral Showhouse, Marineland, Butterfly Garden etc.
By the way, you may also consider coming here in winter.
No boat tours in winter, but frozen Niagara also makes quite an impression.
More pictures and videos are here.