One person mentioned Monarda - will do great in the sun but does not like it's feet wet and it is susceptible to powdery mildew. It can also grow quite tall and may be taller than your sun dial. There is a small variety - Petite Monarda that has a lovely habit and is not bothered by mildew as much. It grows in a small tight mound about 10" high and still has that lovely Monarda spicey smell (the leaves - not the flowers). Summer bloomer.
Echinacia (Cone flower), Rudbeckia (Black Eyed Susan), and Tradescantia (Spiderwort) are some nice choices. Depending on the height of your sun dial, the Echinacia and some variety of Rudbeckia might be too tall - but the Tradescantia is only about 24". BUT be careful of the Tradenscantia as it seeds like crazy!! Tradescantia is a late spring bloomer, the others are summer bloomers.
Another great one that is very well behaved (ie doesn't seed, nice compact clump, never too tall) is Huechera (Coral Bells). One of my favorites and you can actually plant a garden in several variety and have many colors of foliage. The foliage is just as pretty as the flowers. I HIGHLY recommend this one. If you want to see all the wonderful variety of Huechera and Huecherella (a cross), check out one of the country's foremost Huechera breeders at
http://www.terranovanurseries.com/ga...a-c-82_23.html. In fact ... you'll find a lot of different perennial's on their site. You cant buy direct of course, but you'll know what to shop for. Note on the Huechera - some are best in shade, and some in sun. Choose carefully. Also ... pay attention to planting instructions - they don't like to be completely buried - nor do they like to be bothered once they are planted. Summer bloomer.
Iris ... many Iris love to have their feet in water. There are SO many Iris variety you should be able to find the right size so it doesn't dwarf the sun dial. When you have an Iris bed, plant them and leave them alone. They really don't like to be moved and they may not bloom the first year. Don't fret - just leave them alone. Another thing about Iris, they will multiply (and seem to bloom better the "tighter" the corms are), but once they become too crowded you will need to split them - at which point they may decide not to bloom the next season again. Spring bloomers.
Several Sedum varieties should do well too ... and could be the late summer-fall bloomer that rounds out the season. Many varieties to choose from, from medium size mounds to ground covers.