My caveat emptor is that I'm biaised as my favourite city - Vienna - I think fits your bill exactly. I visit Vienna several times every year and think of it as a second home - although if I'm very honest I think of several places as such ......... all of Europe is pretty close to me!

Vienna is easy and inexpensive (if you book now) from LHR and back to AMS. It has a very compact central area that is largely pedestrianised - except it also has one of the best central bus/tram/metro systems of most cities. The small buses in the central area are even allowed over some of the pedistrain zones so you will never be far from a bus to get you back or to simply sight see from - and a daily ticket allows you on everything. So it's perfect if you want to escape from the walking. It still has the remnents of old fashioned galantry and manners ..... and you'll find for example that if you step off the pavement, cars will stop for you ..... and not sound their horn. Around the central area it has a Ringstrasse (ringroad) where you can see many of the sites and trams (1 +2 and a tourist tram) will take you all the way around. It is also a very safe city. I'd also say - with deference and respect to the other cities - people are polite and cultured and patient - the antithesis of what you'll often find say in Paris. There are Wurstl (sausage) stands to explore - one even will sell you a glass of champagne with your sausage!

It has some of the finest archiatecture of any European city and is particularly suitable for two of your needs ...... the odd sit down and the odd people watch - and that is the tradition of the Viennese coffee house where you can sit and relax with your coffee and cake and a paper (many have English papers) for as long as you want.

There's some great art in Vienna - from Klimt to Breughel and a whole Museum quarter to explore with a museum to meet any of your whims.

So I suggest you do a bit of Vienna googling.

How does that sound?