Take 90 to 290 to 190 (follow signs to Buffalo and Niagara Falls US).
After crossing second bridge on Grand Island take Robert Moses Parkway
(sharp right hand ramp exit just before bottom of bridge) which goes
along the river. Take the American Falls exit which brings you to
Niagara Street and you can only turn right. Go up to the second set
of lights and make a U turn which will bring you to Rainbow Bridge.
This seems to be the quicker and easier way to go.