We've been mainly talking upper layers, but a really nice bottom option is the Peloton 97 zip off bottoms
I purchased a pair after being very impressed with the 97 hoody (5 oz); the zip off part I thought was basically a gimmick. One especially cold morning (on top of the Chinese Wall in the Bob Marshall) I didn't want to take the bottoms off, so started hiking in them. Of course eventually it warmed up and needed to shed them. Thinking crap- shoes off, gaiters off, pants off and then all back on. I remembered the zip off feature, again thinking it wasn't going to work. Pants down, unzip each side, off they come- voila! They actually work and work well.
All for 4.5 oz; I think for a September trip in the mountains there's a better than fair chance they'd see some use and when they aren't needed any longer, zip them off!