Sazova Park is the city's outsized landscaped park, complete with an artificial lake, a children's railway, a pirate-ship playground, a science centre, an aviation pavilion with a couple of decommissioned jets, and a startlingly large replica medieval castle that floods with families on weekends. It is unapologetically themed and a great place if you are travelling with children.
The park is on the western edge of the city and most easily reached by tram or taxi. Plan a full half-day if you are bringing kids. Adults will probably wander it for an hour and move on; the riverside in the city centre is better for grown-up loitering.