Reckon someone will put me in the 'heavy' category for what it's worth, mine is a Sporty's flight bag filled with:
1. Beacon
2. Spare pair of glasses
3. Skydemon GPS
4. Chart(s)
5. Kneeboard
6. Stopwatch
7. POH
8. AFE flight guide
9. HiVis
10. Whizzwheel
11. Pencils and Fine permanent markers
12. Logbook (always try to fill it in same time as the techlog when I land home)
13. Handheld radio (charged)
14. Ruler/protractor combo
15. Lockyears
and finally and embarrassingly 16. Boots nail varnish removing pads - for rubbing out the permanent marker on the chart if required (honest, I don't do my nails when the autopilot is on).
Sickbags, Fuel tester and fuel dipper all live in the aircraft. Though an instructor once advised me to pop a sickbag in your kneeboard during the pre-flight if carrying passengers.
I also have an A4 wallet book for overseas flights which contains all the documents you need, insurance etc, together with a bunch of GA report forms and all the plates of airfields on the Dutch and French coast (in case of weather probs on way home).
Some might consider all this over the top, but what I like about this approach is that when I pick up my flight bag (if I can lift it), I KNOW I've got everything I need. It's a bit of a drive to the airfield and I want to do it once rather than thrice!