For a chair, take whatever is portable. Something height-adjustable is great if you have it, but almost any stable support will work. I used to use a kitchen kick-stool, inelegant but effective. Even a sturdy orange crate is OK, with the added benefit that you can put all your accessories in it on the trip to the observing site. You can live without one unless you have something like a bad back.
Many observers carry a collection of nibbles and munchies to sessions.
Don't forget a water bottle or thermos full of whatever liquid refreshes you, too. Time for a review. The best thing to do is go observing with a group of local amateur astronomers. This way you can take advantage of all the scouting they have already done to find the most convenient observing locations with good horizons, minimal dew, no lights, and no legal access problems. You will often find they have worked out understandings with landowners or park officials to use good observing areas closed to the general public at night.