A rock garden is a great way to fill a sunny space especially if you don't want to devote a lot of water and weeding time growing a regular perennial bed.
Decide (with or without a designer) the general theme of the rock garden. A link below this post will give you ideas of different rock gardens (woodland, alpine, water-themed etc). Think about how the rock garden will blend in with the rest of the yard. Will it be set apart or will you need border plants to tie in the rock garden with your perennials? For instance, if you have a traditional country garden you may want to use plants like pin cushion and dianthus in your rock garden so that the beds compliment each other. Sedums and saxifraga look great in a northwest rock garden alongside a perennial bed filled with bolder plants like canna, hostas and other broad-leafed plants.
Start small if you're a little unsure about your rock garden skills. Think about what the plants will look like after they've grown in around the rocks. Don't go crazy with the rocks-You really don't need that many to achieve a nice, natural look. And try to avoid using too many different kinds of rock or your yard might end up looking like a set from Disneyland. Use self-control and start with a basic basalt or granite or limestone. Remember, the plants should be the stars and the rocks should be the backup singers.