If you ask me about my favourite place in Warsaw and where to admire a sunset, I would say it’s Siekierkowski bridge. I like going there on bike, as on the west side there is a bike path that leads to the bridge through nice green areas. If you don’t have a bike, you can rent one of the city bikes (check Magda’s article about Veturilo bikes). To reach this place you can also take one of the buses and get off at e.g. bus stop Siekierki-Sanktuarium and then walk along the bridge.
I take my blanket and something to eat to spend some time on the picnic at one of the lake by the bike path first. For the sunset, I reach the top of the bridge. It’s the best spot with the most picturesque views of the Vistula River with the ‘Manhattan of Warsaw’ in the distance. You can see the panorama of the capital but you don’t actually feel like you’re in the city. I love watching from that spot the moment when the sun says goodbye to the city and hides behind the famous Palace of Culture and Science and other skyscrapers.
I also love this view from a much lower level – from the level of the Vistula River. There are more and more people who can take you there on their boats or who can rent you a kayak. Just be mindful of the time — it’s forbidden to be on the river when it’s dark ;).