A good pair of gloves, spare tube, tools to remove said tube, a pump or CO2 canister (all can fit in a small/medium seat bag).

And fold out bike rack baskets such as this one -> http://www.mec.ca/AST/ShopMEC/Cyclin...ing-basket.jsp

Panniers are nice, but the times you need one, often you forgot to take one along, so I keep a basket on my left side of the bike rack and put the pannier on the right when I do plan ahead. But when I do not plan for it, the fold out rack and a few bungie cords and I can tie down the occasional groceries etc without a hassle.

Jim