The first that came to mind was the creeping sedum. May I also suggest japanese spurge, ivy, vinca vine, virginia creeper, wild strawberries, creeping cinqifoil, catmint, lamb's ears, trumpet vine etc. I realize that many of the plants I listed are invasive, but if it is an isolated hill that you just want covered in green, then the invasiveness will help get it covered faster. I'm in
zone 5a and I find that even the more invasive plants like virginia creeper and trumpet vine are much tamer here than they would be in the south or warming climates. Our winters usually knock everything to the ground. If you keep the vines trimmed, they will not get out of hand. Also remember that you can use vines and let them sprawl (and with vigorous annual vines like morning glory) let them reseed. The morning glories will give you beautiful rustic
flowers. Another rather invasive plant that you can try is the Chameleon plant with its multi coloured foliage.