Stromovka park is very worth visiting. It's popular but you can find quiet parts as it is huge, and it has a few lakes. The planetarium is here, near the entrance in the park, and is a good diversion. Outside the park and nearby is the Exhibition Grounds, in a few buildings, which I thoroughly recommend as there are very interesting exhibitions for the public on nearly all of the time, and a few really good permanent exhibitions (and a famous big fountain). Take a look at what is on.
The modern art palace of the National Gallery is not far from here and is a great gallery which many tourists miss because it is not near the Old Town, the Castle or Malostranska. It is usually quite quiet and I really recommend seeing it. It could take most of a whole day with a lunch break to visit all of the floors, so one and a half hours is the minimum time I recommend. You can have tea or coffee or a cold drink and snacks there.
The vast majority of people who visit Prague see only the three main areas with maybe also the Jewish Quarter and/or New Town and miss these attractions I mention. But especially if you have been to Prague before, make a point not to miss them. Don't forget the great value of Pension Vltava and recommend it to those who would like a very basic, clean and quiet place to stay with the added bonus of cheap drinks in your room at any hour.
For £7 or £8, have a sauna for a few hours in a basement private day spa near Holesovice station. Another similarly priced sauna in Holesovice is infinit (infinit.cz) which also has a jacuzzi.
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