IMHO if you cross the Niagara Falls crossing just after lunch time you will be fine.

Parking:

Check sites like groupon, teambuy for discounts
OR
Look at sites like Park'n Fly to see if there is any coupon or discount for prepaid booking on-line.

Warning: If your return is a busy time (after school holiday week) it can be a very long wait for the shuttle.

I usually just park at the airports long stay lot and take the monorail train to the terminals. It is a little bit more but the wait for the shuttle bus is the deal killer for me.