I have different stuff on my two bikes:

Both have computers and bottle cages, and I always carry my cell phone

Road bike: small wedge bag with spare tube, patches, minimal tools, etc.

Commuter/ running around town bike: tail light (all the time) and headlight (only when I'll need it)
rear rack
locks
bell
and the trunk bag and grocery panniers as needed

I know I really ought to carry tools/ flat kit in town too, but the only mechanical trouble I've ever had on that bike happened within .25 miles of my house, and most of the time I'm within 4 miles of home... Sometimes I carry the kit, but usually not.