It's a lot of fun having a pannier or two.

This kind is very useful for most of my trips:

http://www.lickbike.com/productpage.aspx?PART_NUM_SUB='3657-00'

It says grocery pannier but anything can go in it. It is much bigger than you think. Just last week I used it for my luggage on my trip to CA. I put in 3 days of clothes, toiletries, and a pair of shoes. Put it on my bike and off I went to the airport. It is a nice carry on.

This one is great, too. It goes on the rack and also holds more than you might think. I bring this to work daily. I fit in my lunch, pump, shirt (tee, , towel, rain cape, lunch utensils, wallett, and cell phone.

http://www.lickbike.com/productpage.aspx?PART_NUM_SUB='3648-01'

(actually, I own the Breezer brands of these. The look functionally identical)

Note that the grocery pannier might or might not hold your laptop. Mine does, but I have to take it out of its case. When I carry my laptop in the case, I just bungee it to the rack. I use two small but thick bungees to afix the carrying handle to the rack. I let the sides flap (which they will when I stand while pumping up a hill). If I'm going short distances, I just put the laptop on my rack--I have a spring loaded clip.

I haven't taken the step of the Ortlieb type pannier, but I'm sure it's nice. If you balk at the price, try a grocery pannier first and see what that does for you. They are great.