My family and I go to Niagara every year and have done just about everything there is to do there at this point although every year they add something new!
Some of our favourite spots are Dufferin Islands - perfect for picnics and relaxing in the shade. If your standing in front of the falls, you would travel right down the main road and it's about 5 minutes from the falls. It's a beautiful park with little trails and ponds and waterfalls. You *can* swim in the waterways but we haven't - that's why we get a hotel with a pool. It's free to go there, the washrooms are very clean and it's a nice way to escape the madness of Clifton Hill!
A few other worthwhile attractions are the Butterfly Conservatory, the Aviary, The Secret Garden (free), Behind the Falls, Maid of the Mist (my dh and dd did this and said it was beyond cool!). The Skylon Tower is pretty neat too if you want the very best skyview of the falls and Niagara. It's costly though. About $19 per person.
Because of the time you are going, the thousands of lilac trees will be in bloom down by the Floral Clock. It's a nice spot for a picnic too. And it's right near the Butterfly Conservatory. If you are driving, most of these are VERY easy to reach - one road here, one road there.
Many of the exhibits I mentioned are Niagara Park attractions and you can purchase passes for many of them for one price. Down by the falls there will be booths and such to get more information.
The wax museums on Clifton Hill are cheesy at best. But it's a tourist destination right? lol
You can expect slightly higher prices for meals in and around the falls. Most have their menus out front and many (not all!) will not take reservations. The later you choose to eat, the longer you will wait. Most have US and Canadian prices on their menus as well.
Parking is on average $10 if you park in a public lot. Not sure if your hotel charges parking. There is a shuttle service to all of the Niagara Parks attractions which can be very handy. There is private parking all over the place, but often the people look a little shifty. We usually stay on Clifton Hill and walk as much as we can.
There are fireworks on the weekends too which are really great. They are over the falls and there is a giant park right in front of the falls where you can sit and enjoy. They usually have live music then as well.
There are also a few new waterparks to check out too. Wolf Lodge seems to be the most popular!
At that time of year, it's quite warm but the nights in the falls can be very cool. And very moist! lol You'll be fine with shorts in the day (and good walking shoes!) and pants and a light jacket in the evening.
Hope I didn't overwhelm you with info! Have a wonderful trip!