Sarah
Here's a photo tutorial on how to add a simple crocheted embellishment to knitting. I just think this is such an easy way to add some pretty touches to plain stockinette -- and as I said yesterday, it's pretty easy to undo, so experiment!
You'll need something to embellish (stockinette seems to work best), a contrast yarn, and a crochet hook that works with your knitting and your embellishment yarn. You can be pretty flexible with the crochet hook size -- as long as it's big enough to grab your embellishment yarn, but not so ginormous as to permanently disfigure the knitting it has to pass through, you'll be fine.
Poke the head of your hook through your knitted fabric and grab your embellishment yarn, making sure to leave a tail long enough to weave in later. 
Bring the loop of embellishment yarn through the fabric to the front.
Leaving the first loop on the neck of the crochet hook, poke the head back through the fabric (in whatever direction you want your embellishment to go) and grab another loop of your working length of embellishment yarn.
Bring it back to the front. There are now two loops on the neck of the hook.
Bring the second loop through the first. 
Continue in this manner to create embellishment. The great thing about this is the ease with which you can move in any direction, "drawing" on your knitting -- and if you decide you don't like how it's going, to rip back all you need to do is remove the hook and tug on the working end of the yarn. To finish, simply cut the yarn, pull the tail end through to the front, over the last loop, and back through to the wrong side of the work. 
Not your thing? I'll be doing a duplicate stitch tutorial next. Stay tuned!