I live in Amsterdam. It's nice to visit for a few days, but it's small so three days is more than enough. It's a 10/15 minute train ride from Schiphol airport, so connectivity is awesome. The inner city is the only thing which really matters, the rest is just.. well.. neighborhoods. There's definitely some more to see in the Netherlands, but for me, as a native, it's all just kind of boring and most of it (especially further down the country) is being overflown by Chinese tourists.
From Amsterdam you can very easily take a highspeed train to Paris (takes three hours i think) or a 'normal' train to Berlin (5/6 hours). If I'd had to choose cities to visit in Europe I'd consider the following: London (you can also take a train from Amsterdam to London, but you have to switch trains in Antwerpen (Belgium) i think and also London is fucking expensive, Paris (same, but it just has it's charms), Budapest (crowned as the most beautiful city of Europe and relatively cheap), Prague (beautiful, cozy and also cheap). Barcelona is too crowded (way too much tourism). Italy might be worth looking into as well since it's countryside is beautiful and it has a bunch of cities worth checking out (i.e. Rome and Milan).
Nature-wise i'd say Swiss is the most beautiful country in Europe. So if that's what you're looking for over city trips i'd highly recommend checking that out. Austria is very nice as well.
If you're planning to do Russia it might be a good idea to take that long ass trainride from Moscow to China/Korea.
