This morning I had to carry a smallish, lightweight box to town. Its cubical shape made it awkward for a pannier or to fasten to the top of my rack pack. It did fit in this handy dandy day pack I got at Eddie Bauer a couple of years ago.
The handiest dandiest feature is that the pack stuffs into its own bottom pocket.
Stuffed, the pack is easy to slip into your luggage or otherwise tote along when you think you might want it. I also like how it reminds me of a calzone. Although I would not eat a green calzone, cyclists are always hungry. It doesn't take much to remind us of food.