I'm a pretty light packer. I have 3 seat bags, but usually use the medium one. I carry a tube, sometimes 2, and the rest of the kit (CO2 cartridges, etc). I keep a couple of bandaids, sometimes a single-use sunscreen packet.

In my jersey pockets: cell phone, a few bucks, debit card, a few sticks of gum, and some snacks: usually shot blocks, Luna bar. Sometimes a Nuun tablet wrapped in foil, or a plastic baggie with powdered electrolyte mix.

I sometimes use a Bento bag, but have had some bad experiences with getting my shorts damaged from the velcro. On my longer rides, I have carried extra energy bars, a bag of fruit and nuts, GU in the Bento bag or in my pockets. I sometimes carry a small camera or mini-binoculars in the bag or in my pockets. If I need more food than that, I'll buy it on the way (a banana, a gatorade, etc)

Rarely do I need a lot of extra layers even in the winter, but I am able to pack arm warmers in jersey pockets. I can usually get away with wearing a pair of running knickers over my shorts and if I need to, can roll them up and put them into my larger seat bag.