Since I got my Nokia N900, I find that I rarely need my laptop. The N900 gives me access to email, web sites, and reads pdf, doc, ppt, and xls files. It also gives me GPS, skype calls, and cellular phone calls, of course. I think any good android phone, iphone, or maemo phone will give you 90% of what a laptop can.

Also, I rode with a backpack my first year, but switched to a set of rear panniers. The panniers are much easier for hauling clothes, lunch, and tools.

For lightweight tools, I carry only the following: Park Multitool MT-1, spoke wrench, plastic tire levers, spare tube, Topeak Road Morph pump, and a Park VP-1 patch kit. That stuff will allow me to to change a flat and make minor repairs and adjustments. I've never needed anything else, and only had to call for a ride home once in the past 3 years. The ride home was needed because the weather was freezing and my hands got numb trying to change a flat