Depends on what I'm doing.
For example, a weekend walking in a National Park as part of a group, I'll generally have a "sports bag" type duffel bag for "long term storage" (ie: everything lives in there, with stuff being removed as required), plus a small hiking rucksack (something like 35L) for everything I need when out (which is basically just food and drink), so i actually have quite a bit of spare space. For a weekend I would take a set of "normal clothes", set of outdoor clothes with spare shirt. The space killer is food (need to bring everything for the weekend), sleeping bag, roll mat, coat, walking boots and towel (primarily to use as a pillow, but also gives the option of a shower).
Next weekend I'm doing Birmingham to London to Fort William (sleeper) to Birmingham. I'd happily do that (a day and a half) with just the contents of my pockets if necessary, but I will be bringing a laptop and DSLR so a rucksack it is.
Depending on the length and planned activities, I'll do an Interrail trip with either a duffel back and smaller rucksack, or a large hiking rucksack