I am with Dottie that you would do well to plant next fall (mid-September to late October in your zone) to allow root growth but have less stress on the plants due to shorter, cooler days. Regardless of what you plant, it will need regular water until the ground freezes. This season you can evaluate the site and plan so that you are ready come fall. I also agree with Dottie about voles - you could ask on the perennial forum or on the shrub forum (whatever type of plant you end up with) for suggestions that seem less attractive to voles.

Most types of deodar cedar (Cedrus deodara) are hardy to zone 7, so not an option for you, though there are some types rated for colder temperatures such as 'Kashmir', 'Shalimar' and 'Polar Winter'. I'd check around at local garden centers, botanical gardens, and arboretums to see if anyone has local information as to how they do before planting them. It's tough having a borderline hardy tree that does well for several years and then is half-killed during a particularly cold winter.

My inclination would be to plant native plants or ones that you are sure don't have escapist tendencies since you are on the edge woodlands, and plants such as ivy or vinca can grow into the woods, disrupting the native plants. You may find that you have already existing woodland spring wildflowers when you go into the woods in the spring. (Be aware that spring is the worst time for ticks, though they around all of the warm season, so get in the habit of doing a careful check of clothes whenever you have been out in the yard, and of your entire body at least once a day if you have been outside at all. It takes a few minutes, but I include it in my bedtime routine and haven't had an embedded tick yet, though I find them on my skin and clothes often.)

Light levels: If you plant the slope with typical meadow wildflowers, you may have mixed results, depending on the amount of sun you have. Something like Rudbeckia is really a sun flower, and so may not do particularly well there. I would start taking photos at 8:00, 10, noon, 2, and 4:00. Do this at the spring solstice (mid-March, 1st day of spring), and about once a month up until the summer solstice (mid-June, 1st day of summer.) This will give you a good idea of how much sun you actually get there. If you want to try Rudbeckia, I'd go ahead and do that this spring since it will establish better with less established grass. By planting time, you may have a pretty good idea of how much sun the area gets.

Water and nutrients: You might want to ID the trees along the top of the slope; depending on what you have, they may send out more or fewer shallow roots into the slope area to compete with whatever you plant. I say this because they look to me like red maples, which have fast-growing shallow roots and will compete for water and nutrients with whatever you plant. So the sooner you plant, the sooner what you plant gets established enough to be able to compete with the red maples. If you have oaks, they won't be an issue since their roots are deeper. Watch the slope and see how water levels fluctuate over the year. If it is a recently cut bank, the soil may actually be subsoil and benefit from adding organic matter to the surface, or there may be springs that will cause problems if not addressed. I'd also get a soil test done (Cornell U does them among other places) to see what will grow well in your soil. For instance, that might be a lovely place for some rhododendrons, but only if you have acid soil.

Mowing: If you end up deciding to keep this meadow, it will need mowing 2-4 times a year with a brush mower to keep out woody plants. In my area those woodies include sweet fern, red maple, staghorn, sumac, white pine, black cherry, all of which like open areas for their seedlings. Seeds will blow in or be brought in by animals, and some of them will start growing, often including non-native invasives such as buckthorn, bittersweet, barberry, burning bush, and Japanese honeysuckle. In the northeast and Great Lakes area, unmowed areas will always turn into forest eventually, so you want to mow to prevent the process from getting started. In our field we time mowings to allow the ground-nesting birds' chicks to hatch undisturbed (turkeys are our most frequent ground nesters), and to allow certain plants to bloom, but for a chunk of time it is just mowed field. You can time one of your mowings for early fall just before the leaves change color, which will allow flower seeds to ripen but prevent baby trees from storing nutrients. Your landscaper may have access to a brush mower to do this for you. Regardless, the part you keep for sledding will need either regular mowed grass or the periodic

Consider a mixed shrub border for the less steep part of the slope. I have one that is heavily mulched or groundcovers between plants and edged to keep out grass, and I only spend a few hours a year (perhaps an hour a month) weeding it, and it's probably about 15'x70'. It provides blooms, varied summer foliage, winter evergreens and berries, and colorful fall foliage for very little maintenance work. In the late fall or early spring I use hedge clippers to whack off the perennials and clematis to ground level. This one is full sun, but I have others in less than half day sun with a mix of shrubs and a few perennials, also edged and heavily mulched. This one is about 5 years old.

Fall

Spring

early Winter

So you have lots of options, but specific plant recommendations depend on your goals, your soil, and your sun levels.