My son is flying Sep. 21 KL1352 PRG-AMS connecting to KL0621 AMS-ATL. He'd like to take advantage of the 6 hour layover and see downtown Amsterdam. Can anyone provide any advice on the pros/cons of this plan? There should be no customs PRG-AMS as they are both Schengen. His bag I assume will be checked through to ATL since it is all on KLM. So presumably he would only need to arrive back at the airport in time to clear Security and get to the gate, maybe 2 hours? He is an adult but not an experienced traveler, so getting stuck in AMS would not be good. Appreciate any advice you can offer.

It would be a quick trip, but I would recommend going to go to the downtown area using public trains (ns.nl). Given it is shengen to non shengen, I would recommend him to be back at schiphol at least 1h30 before departure to go through customs etc, so he should have at least 3 hours (probably 4) in amsterdam, which is enough to get a quick flavour of the city.

Is OP thinking about a DIY trip into Amsterdam or the official airport bus tours (IIRC free) for passengers with sufficiently long connections that I believe were offered by AMS among other major airports preCOVID-19?

BTW, will there be any testing, quarantine, etc. requirements? Can we assume that since he's coming from Prague, entry into the Netherlands (Schengen) will not be an issue?

I can answer the second: in september I would be highly surprised if there were any testing requirements, most regulations were dropped in the last few weeks anyways

There are no testing requirements from EU anyways if vaccinated.

It takes about 15 minutes by train to get from Schiphol to the Central Station so a 6-hour layover is easily enough to do some basic sight-seeing.

While there may not be much of covid testing requirements currently, I do recommend checking that again close to departure. Things may change on that front. Wouldn't be the first time in this pandemic that we thought to be almost done and that things could only get better from now

I'd try to be back at the airport at least 2 hours prior to departure, especially if not having status and flying economy. Lines could be longer than expected.

Thanks all for the advice. He wants to do a DIY trip into town and he will return 2 hours ahead of flight time.

The train is fast and reliable, the plan should work

Echoing the previous comments, make sure he uses the train.

In this case, i would also recommend to take the train into Amsterdam. When exiting customs, follow the signs for the trains. Tickets can be bought at the machines at Schiphol Plaza, then go down to the platform where the next train for Amsterdam Centraal leaves from. Obviously, your son doesn't have that much time, so first thing i would do after arriving at Amsterdam Centraal Station is heading to one of the canal cruise companies in front of the station and take the next available general canal cruise. They last usually 1 hr - 1 hr 15 mins, and give you an excellent tour of downtown Amsterdam for a very reasonable price. For a first time visitor to Amsterdam, even if wanting DIY, a canal cruise is a must. There are quite a few companies competing but that should not be an issue. Take the first one you see. Afterwards, there would be time to wander around a bit in the old town and have lunch, then back on the train to the airport