Of course the Falls because it is usually misty there anyway. You also have the Casinos in Buffalo and NF. The Fashion Outlets in Niagara Falls is a huge mostly indoor complex. The Albright-Knox Art Museum is a gem and the Elmwood Avenue area is nice to go through to get there. Another idea is the Niagara Wine trail. South of Downtown Buffalo there is the Botanical Gardens which has an indoor dome and greenhouses and the impressive Basilica of Our Lady of Victory is nearby that. Another idea is the Lockport Cave tour and stop in at the New York Beer Project on the way there.