For day hiking when I am on a bike tour, I use a lightweight fanny pack. It is an old one (1980s vintage?) with some leather patches on it for extra straps, I can strap a jacket on the outside of the fanny pack. Folds up to almost nothing in teh bottom of the pannier and only weighs 300 grams.
I gave my sister a backpack for Christmas, she wanted one that folded up small but could be used for travel. I got her one of these:
Sea to Summit Ultra-Sil Daypack - REI.com
I have not tried it but she looked at it in the store and decided she wanted one for travel and sightseeing.