I've got a couple of pairs of merino arm warmers and pretty much always stuff a pair in the back pocket of my jersey even in summer - they're light enough to carry and a simple way of adding a layer if it gets a bit cool, or if I need to keep the sun off my delicate skin.

I also have some more heavy duty ones that I use for layering during the winter.
Knee warmers, I was a bit dubious but now I've tried them they're an essential bit of autumn / winter kit and do a good job of keeping the chills out of my creaky knees when it's not cold enough for full bib tights, or when setting off early and expecting it to warm up later as they're easily removed.