I have recently finished these socks for my niece with my leftover Poprocks yarn. As something special I have added small hearts into the heel. The heels are knit with slipped stitches, but the hearts are knit with regular stockinette stitch, which makes them slightly raised. I think they are very cute.
I have also tried a new cast-on recommended in The Handknitter’s Handbook. You cast-on using the tubular cast-on and on the first row you knit and purl almost like you normally would, you just twist the purl stitches. I think it works well.
Here is the chart, if you want to add hearts to your heels:
And here you can find it in a pdf document for easy printing.