This is a highly recommendable approximately 30k bike tour along the river Vltava passing the Prague zoo and the Troja (part of the National museum) with its vineyards. Very relaxing and safe to ride (mostly bike lanes without any car traffic).
I first took this trip some six years ago, on an old “Eska” folding bike (Czechoslovakia, built in the early 80s), 20″ wheels and fixed gear. It took me just over three hours to complete the 30k stroll, including many short breaks to take pictures and the ferry boat trip to cross the river from Klecánky to Roztoky. Ticket price was Czk 20 back then (0.74 Eur), now it should be around Czk 30.
There are various food and refreshments stands by the bike on the eastern side of the river, some selling the excellent beer from the nearby Unětice brewery. If you have enough time, you can pay the brewery a visit, it’s just a few kilomters uphill from Roztoky, via the idyllic “silent valley” (Tiché údolí).