Amsterdam for a couple in their 50's

Amsterdam is absolutely perfect for what your looking for. There is a very strong cafe culture. Just look up the term “brown cafe Amsterdam” to see what I mean. In addition to that the city is ideal for wandering around aimlessly, pausing at a canalside terrace etc etc.

The only downside are your dates. From September 11 through 14 there is a huge convention in Amsterdam; the International Broadcasting Convention.
The convention takes place in a convention center just on the outskirts of the city center, but hotel rates throughout the entire city skyrocket during the time frame of the convention as well as the days before and after the convention. Before you book your plane tickets I would look at hotel rates first.

If staying in a somewhat residential area, Amsterdam is lovely for what you at describing, but the train system is so great you could stay in a different town. I loved my day in Leiden.

Absolutely Amsterdam will meet your wishes! And you can pop on a short train ride to Haarlem! We loved the Corrie ten Boom House - WWII location where Jews were hidden by family. Beautiful smaller city.
Enjoy - I'm jealous. Amsterdam is one of my favorite places. And consider reading Russell Shorto's book: Amsterdam. I loved it and can't wait to get back to experience the city is a new way.

Due to the costs and crowds, you might consider staying elsewhere until the convention is over. You'll still have plenty of time in Amsterdam. It's a GREAT place to visit and easy to avoid the red light district and party scene, which is definitely a thing. Take a look at https://sweetshotel.amsterdam to see if that's your type of place. It's a dispersed "hotel" housed in retired bridge houses. I haven't stayed there because most of the units haven't suited my needs. For meals, I recommend Zwaantje (http://zwaantje-restaurant.nl/?lang=en), near Anne Frank House. Also, Sampurna (https://www.sampurna.com) or other restaurant offering rijsttafel (several recommendations on the Forum). I have twice enjoyed an evening canal tour with Those Dam Boat Guys. It's BYOB. You can get picnic food at https://www.sterkstaaltje.com. Pro tip: if you're on the last trip of the night and are offered the option to go to the Marina, take it. There is a very good bar nearby for bitterballen (https://www.blaauwhooft.nl).

To add to G3rryCee's suggestion, my husband and I stayed at two of the Sweets Hotel units and loved them.

Hey, thanks everyone. I'm glad there seems to be a consensus in favor of Amsterdam.

I did noticed that the hotel prices are insane. I started off thinking I'd look for a suite. We're booking the airfare on points so I figured I could afford to splurge on the hotel. Well, let's just say I was quickly disabused of that idea! The prices are similar to Paris, London, or Manhattan. In fact, I think I could get a better deal in one of those places. I won't be deterred by that. I'll have to back off the suite idea though.

I do have Haarlem and Utrecht on my radar for day trips. I don't want too much movement. Truth be told, my wife started off this process by suggesting we go to Florida for the week. She wanted to sit by the pool and relax. That will peak hurricane season so we decided not to chance it. It was a bit of a stretch for me to convince her to head to Europe again. Like I said, we just returned from France. So I will keep this slower paced than our usual. Hence, one hotel for the entire week.

I just downloaded the Shorto book. Thanks.

I'm familiar with the Sweet Hotels. I watched a YouTube video on one. They look cool but it's not for me on this trip. I'm looking for something conventional, quiet, that definitely serves breakfast. Neither of us favor searching for breakfast when we're abroad. We like to go to the lobby, eat, drink coffee, and hit the road for the day.

Thank you for all the suggestions. I definitely want to do a boat ride so I'll go with Those Dam Boat Guys. Cool.