Reykjavik is one of the lucky cities that have their own lake. And this one is even situated right at the city centre. Chances that you would visit the city and never even spot it are really slim since it is located right next to the City Hall and the Free Church in Reykjavík (both very popular places to visit).
Feed the birds, go jogging or cycling or just enjoy the tranquil atmosphere here. The Hljómskálagarðurinn Park (full of places to sit down and statues to marvel on) is adjacent to the lake. An ideal place for a stroll!
Wikipedia https://en.wikipedia.org/wiki/Tjörnin
Address Reykjavík, Iceland
Coordinates 64°8'40.125" N -21°56'31.666" E