I am also looking in to panniers and or a trunk bag.

For riding to work in the past I have a very small lightweight backpack that I could carry on the rack with bungy cords. I could stuff whatever I needed to carry to work, including a thermos of coffee, lunch, a clean shirt and underwear, clothes, jacket, arm warmers, whatever layers I might need for the ride home. And bare minimum = small wallet, glasses, phones (personal cell, and work cell). It fits but as the weather got cooler and the clothes a little bulkier, it was harder to stuff the stuff into the backpack. I can wear jeans or khaki pants for work every day, and usually a polo shirt - w/company logo. So I keep some at work and when I drive I can swap out the clothes at work.

So I've been looking at rack trunks and some might be big enough. I don't carry a laptop so it might work. I've found some with drop down panniers - sort of like elephant ears that you could put extra stuff, esp if you stopped at the store on the way home. Trek has one like that, and so does, oh I forget, but it might have been on the REI website, or Jaand maybe. I will probably buy an expandable trunk bag and see how that goes for work. Then for grocery shopping I'll get a set of real panniers I think.

I need to query the participants for the C&O Canal trip about what to be packing for that, or if we can con someone into accompanying us in a car to carry our gear for us.

It looks like Ortleib and Arkel make some pretty nice stuff, but none of the LBS near me have anything like that in the store.