Need help deciding on what to plant in my rock garden with full sun...

wildones.org/land/wibirdpl.html

You could do some hostas, phlox, asters, balloon flowers, mums, and coral bells around the fountain. I would probably also plant some taller flowers in with those, just keep them more spaced out so that they don't hide the fountain, but do give it a little bit more excitement.

Make it an herb garden

Hen and chicks grows just about everyone and proliferates from year to year.

If it's shady I would do hostas they come in all different colors ...if it's sun I love coneflowers they are very hardy and come in colors from reds oranges pink white etc...I'm in Wisconsin too.

Beware of tansy and mints, they are very invasive. They just about ruined my rock garden.

Both zinnias and snap dragons come in tall and short varieties. Very easy to grow and beautiful color combinations are available. I'm in MN so we're about the same zone. You shouldn't have an issue with these at all in full sun