A lightweight, upright bike is a valid style, not because they go faster but they are easier to handle and lift and generally more fun than heavier bikes.
Cruising tyres don't need to be skinny, you can get all the high tech fabrication in a wider style. My Schwalbe Big Apple tyres are not fast but they are efficient at cruising speed.
Cruiser style bikes are not very efficient, OK for a couple of miles, eating an ice cream much not much more. Flat-bar road bikes such as Specialized Sirrus are very common.
You can opt for a hub gear such as Shimano Alfine.
Breezer Bikes make some good lightweight city style machines.