As a broad rule of thumb, any bulbs can be planted during the fall - onion sets and garlics do very well for us when planted this way in usda zone 5 / Trewartha Dca. The same goes for things like daffodils, irises, and other bulbing plants. Trees, shrubs, and perennial herbaceous plants will also benefit from fall planting, and come into the following growing season with a more extensive root system for it. We've also had success with things like kale, chard, and other hardyish cut-and-come-again leafy greens, so that might be worth experimenting with.

True summer veggies - peppers, tomatoes, anything that melts at the first frost - I don't start until late winter or early spring (and those are started inside). Annual summer flowers get pretty much the same treatment, especially any that have a relatively short time to maturity. You could work those seeds into areas during the fall, but you'll have a wider range of results and less direct control (but chaos gardening is cool).