Panniers. Normally just the one middling sized one (Endura, or
Altura, can't remember which), in which I carry rainlegs, tools, lights etc. On Mondays and Fridays I carry my laptop into and home from work, so I have my Altura laptop pannier as well. On Tuesdays, when I go riding, I have my other middling pannier with riding hat, chaps and crop. When I have to take the compost caddy to the allotment, I take the riding kit out, and put the caddy in.
I also have a Carradice SuperC which I take when I have a lot to carry - it's very capacious (I have to watch out for the weight, rather than it being full up).
Going on holiday, I have both panniers and a bar bag.