after dinner stroll
I agree with FIFO's suggestions, especially an after dinner drink at 230 5th. I would also add that you can walk south and east towards Park Ave, and then meander around Gramercy Park and Irving Place after. There's also Curry Hill just a block over, after I've had a good meal I am often in the mood to browse for food related stuff, and Curry Hill makes some excellent window shopping.
It is right next to Madison Sq Park which is nice to walk around - there is a dog run so it can be noisy on that side of the park (east). Then there is the Flatiron bldg and a view of the Empire St Bldg from 5th/Bwy. Not much by way of people watching, although you could head up to 230 5th for an after dinner drink at the bar. It is rooftop (with an indoor lounge) so views are nice.