On the flight back, we also had a whole day in Warsaw. Unfortunately, I didn't get enough sleep on the plane (unlike the flight there), and I hadn't planned in advance what I would do there, but it didn't turn out too bad.
We arrived at around 6 in the morning. We hung around Chopin Airport for a bit, which is bad because it's almost impossible to find power outlets there - I saw three in the whole airport, and they're far from the seats. I plugged my iPhone charger into one of them - a security guard came up and asked if it was my device. Upon getting a positive answer, he said that everything was fine. Respect — he didn't stir up any prohibitive bureaucracy for no reason, but he did his job.
While riding the bus, we got to talking a bit with a Belarusian girl who lives in Warsaw with her husband. She kind of talked to us rather reluctantly (maybe it just seemed that way to me after the endless Vietnamese friendliness), but she tipped us off that we needed to get off at the "center" stop, which is not far from the old town. Near this stop, there is an excellent shopping mall with a large selection of eateries (which was relevant for us). The soup at the eatery was excellent, and everyone liked it very much, unlike the cafeteria from last time.