You've got all sorts of options to carry things by bike.

1. A small backpack is great for a lot of day-to-day items, provided you don't have to go too far. Mountain Equipment Co-op (mec.ca) sells one that's designed for cycling. It's a little narrower than a standard pack, so it's easier to turn your head and see behind you. If you're looking at packs, also look at courier bags, which provide another way of carrying items on your person instead of on your bike.

2. A rack trunk works wonderfully if you've got some small items to carry, but look around before you go this route. A lot of rack trunks won't hold one grocery bag worth of goods, but there are some great rack trunks with larger capacity.

3. A handlebar bag typically has less room than a rack trunk. You will have the advantage of being able to remove a handlebar bag a little faster than a rack trunk. It's a good solution if you're carrying small items you need with you all the time. It's not so good on a grocery run unless you're just buying a few small things.

4. Panniers are great for increased capacity and for hauling items over longer distances. You'll usually be able to unhook panniers quickly.

5. A trailer gives increased capacity, but it also has its own set of issues. There's usually a lot of information about trailers in the utility cycling section.

I personally like using the pack for shorter trips in town. I seldom carry more than I can take in a pack and I like the convenience of having the bag with me. However, if I had to ride farther, I'd use panniers. I already use them for touring where they work extremely well.