It's best for flowering purposes to move fall bloomers in the spring and spring bloomers in the fall. I find that as long as I don't move plants in the heat of the summer, they all tend to transition very well to their new homes no matter what time I move them. Just don't let the soil dry out so they have a chance to establish themselves.

As for low maintanence plants I would suggest the following:

Daylilies. You will need to cut back dead foliage once it is killed by frost, but it's a quick and easy job. You can get daylilies in an assortment of colors and designs.

'Becky' shasta daisies are great bloomers. Of the many types of shastas I have, these are still my favorites. They have very healthy, neat foliage. They will need to be cut back in the fall as well. Again, just cut down the spent flowers to the green foliage at the base of the plant. Great for cutting as well. Usually easy to find in garden centers.

'Blue Spire' russian sage is quite beautiful and handles tons of sun very well. With russian sage you could cut it down to about 8 inches tall once buds start to form in the spring and leave it alone for the rest of the year. Bees love it, so if you want to attract pollinators this is a great choice.

Purple Coneflowers (Echinacea purpurea)- There are a bunch of fancy new colors out there, but the most healthy, low maintenance coneflowers are the species version (echinacea purpurea), 'Magnus', and 'White Swan' coneflowers. My favorite is 'White Swan'. It has beautiful white petals and a greenish colored eye. I just adore the color contrast between the petals, foliage, and eye. This is a great option if you don't like the pinkish purple coneflowers. I have many of the newer varieties out there and none can compare to the older versions in terms of health and vigor. You can leave the seed heads stand through winter for goldfinches to eat. In early spring just cut them to the ground. New foliage and stalks will emerge when the weather warms up. A very easy to grow and low maintanence plant for full sun. Typically easy to find in garden centers.

'Autumn Joy' sedum is a beautiful and easy to find plant. It blooms in late summer to fall. Butterflies love it. You can leave it stand through winter and cut the branches to the ground in Spring as well.

I can't really think of any easier plants than those. They do require a little clean up once a year, but it's worth it for what they provide in long bloom season and overall plant health.

As for your hydrangeas, they appreciate a break from the hot sun. Either you could water them much more frequently to help prevent wilting (a pain), move them to a shadier spot or spot with just morning sun, or plant a larger plant near it to help block some of the sun during the hottest parts of the day.

A butterfly bush could help shield your hydrangea while providing you with tons of summer blooms. These look and bloom their best when you cut them down to about 8 inches in the spring when new buds appear. Cut just above a new bud, and they'll shoot up in no time. Butterflies, bees, and hummingbirds love them. I love the colors of 'Royal Red' (more of a purple with pink highlights) and 'Pink Delight'. 'Royal Red is about 8' tall and 6' wide. 'Pink Delight' is about 10' tall and 6-8' wide. Big but you definitely get tons of blooms for your buck.

I hope this helps, and good luck with your new garden!
Kim

Here is a link that might be useful: Kim's PA Garden

What Kim said!
I have also seen a lot of those "landscape roses" growing well around here, I think "Knockout" is the name of most of them.
Nurseries nowdays are carrying a plethora of small evergreens to, to bring some winter interest.
Harder to come by for some reason but easy peasy to grow are the salvias (lots of other sages in red, blue, purple and coral).
Another fun easy care perrenial is coreopsis.

Congratulations on your new garden! There are soooo many possibilities. If you're in the Philadelphia area, try going to one of our excellent arboreta or botanical gardens and see at what is growing in similar conditions that you like. If you're near Swarthmore U/Scott Arboretum or near the Morris Arboretum, they'd be good places to start.

Also, look through Pennsylvania Department of Conservation and Natural Resources' (DCNR) list of invasive and potentially invasive plants for PA, and avoid planting those. There are so many wonderful alternatives to the plants that are threatening our ecology.

Here is a link that might be useful: Pennsylvania Invasive Plants

Thank you all very much for your suggestions! We have completed the garden for this season -- transplanting the blue caesar irises, a spirea, and a mophead hydrangea that wasn't getting enough sun where it was before. Unfortunately, it's getting too much sun now, so I don't know whether it will acclimate. I have added a crape myrtle, some blue salvia, neon sedum, celosia, black-eyed susan, and agastache. I'll also be planting some tulips and daffodils for next year. We have a bench, bird bath, and two bird feeders (that were added after this photo was taken). Yesterday, the birds finally discovered the seed feeder. Don't know when then hummers will show up.

Here's the before picture:
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And the after picture:
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And the bees love all the new flowers!
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Fantastic job! It is so warm and welcoming. If I was walking by I wouldn't pay much attention to the yard in the first picture. I would definitely slow down to take a look at the new and improved landscape! You should be proud!

Here is a link that might be useful: Kim's Garden

Thanks, Kim! I'm looking forward to seeing it next spring when it's really thriving! This is a project I've been wanting to do for a few years. I'm so glad it worked out as well as I had hoped.

Fantastic job! It is so warm and welcoming. If I was walking by I wouldn't pay much attention to the yard in the first picture. I would definitely slow down to take a look at the new and improved landscape! You should be proud!

Here is a link that might be useful: Kim's Garden