There is usually ample seating inside Sunshine Seasons, if you want to be more out of the way, the "winter season" section over by The Land is less used. Honestly, I don't think it would be a problem, if for any reason you feel weird about it, go buy a drink from Sunshine Seasons and enjoy the free refills! Some of your best memories at Disney can be the little things like this, I say enjoy yourself. It's not like you are bringing in your food to a Table Service. There are also seats lining the walk-way up to the Land building, with trees and some little bit of shade.
In future world there are a few trees and there is grass, but I wouldn't recommend going into the lawn, it is usually bad form to walk over grass or sit on the grass. If everyone did it, the lawn would be wrecked. I recall there being some areas with benches around Mission Space/Test Track.