What fun to start a new garden, I actually filled in a 24 foot round pool area several years ago and it's one of my favorites. I used to call it my circle garden, but have learned it can actually be called a rain garden as it's lower than the rest of my property and actually holds alot of runoff from storms and keeps that water our of the local river. I'm zone 5 and some perennials I can suggest would be balloon flower(platycodon),(comes in white, pink, and blue)feverfew virgo,white,maybe some bleeding heart, catanache (cupids dart)white or blue. Rose campion for really intense pink. Jacobs ladder(polemonium) falls into that 2' height,purple or white. There are several perennial geraniums that would work. A few columbines lots of different colors there. You could also put in some white blooming bulbs for early spring, iris would be good too. I could go on and on whith what works in my zone. Good Luck.

Phlox come in many colors, bloom at different times. Newer ones are better resistant to the powdery mildew old Phloxes got. Creeping Phlox blooms quite early, spectacular color, low growing. Ageratum is pretty too, low growing, in white, blue, often reseeds. Cosmos is a great plant, ferny foliage, flowers in shades of pink and white, later in the summer. Often will reseed for the next summer. Might be a little tall, but a great plant, an annual along with the Ageratum. My Dianthus are great for color, come back each year, many choices of color for early season bloom. May keep blooming if kept deadheaded.

You will want to consider bloom times, so you have something going all season. Clematis is a climbing vine, takes a little time to get started, but very rewarding. Many colors, shapes of flowers, bloom times, to choose from. If you got an early bloomer, it could make the color until the Morning Glories got going later on. Some small shrubs could be anchors, changing the annual flowers around them every year. Small Hydrangeas could help you there, along with other flowering shrubs.

I like sedum 'Frosty morn'; in addition to pale pink flowers, it has a nice form and variegated foliage that look good all season.

Phlox 'Nora Leigh' also has bright variegated foliage and soft pink flowers that last for a very long time.

Agree with goodhors about the clematis. They are fantastic.

For annuals, consider easy cosmos and cleome. Long lasting, easy care, pink and white, self seeding.

Have fun!

I have a petite monarda that is clear pink. I can't get to the label right now (it's under snow), but this was less than 1 foot high in the first season.

Ageratum, as mentioned, is a pretty pastel blue in perennial form, and there are white, blue and pink annual forms.

I like my hardy geranium 'Brookside' and 'Rozanne' but the rabbits do go after those.

Spirea 'Neon Flash' is a pretty, short shrub with dark green foliage. The blooms are pink/deep pink. I use it with coneflowers, ageratum, and salvia.

If nepeta 'Walkers Low' isn't too big, it's a great one to use. I also like scabiosa, but don't know if that works for your zone.

Cameron

Depending on where your zone 7 garden is, I would caution you about some of the mentioned ideas. Beautiful plants, but don't like my Deep South zone 7 garden at all:
phlox paniculata (I have yet to grow a truly successful one),
feverfew,
bleeding heart,
catananche,
Jacob's ladder, and
sedums have all been anything from disappointing to complete disasters.

That's not to say there aren't plenty of winners that will grow here:
Black and Blue Salvia,
Salvia farinacea,
Mystic spires Salvia,
Salvia lyrata,
Salvia chaemydroides,
Geranium Rozanne (blue in spring, pink when the heat turns up, but blooms beautifully regardless),
Peachy's Pick Stokesia,
Crinums,
Rainlilies (zephyranthes, not habranthus),
phlox divaricata (my favorite spring blooming perennial), Brazos penstemon (penstemon tenuis),
Verbena Snow Flurry,
hellebores,
daylilies,
agapanthus.

I would strongly encourage you to use at least a few evergreen shrubs so you'll have winter interest, early spring bulbs like daffodils, and consider mixing in some bright colors like yellows, oranges, or reds, at least in the summer. Our summer heat really washes out pastels and they just seem to disappear.

Good annuals are:
profusion zinnias,
zinnia angustifolia,
torenias,
cosmos (though the foliage of the tall ones tends to be leggy),
gomphrena,
gloriosa daisies, and of course,
pansies through fall and winter.