A touristy thing that's definitely worth doing is visiting PKiN, the Palace of Culture, and going up to the viewing platform on the ~32nd floor.
If you're flying to Modlin and have an interest in military history, you could try to compensate for the horrendous airport – it's a bit like Luton but half the size, four times as cramped, eight times as far away from the city it's supposed to serve – by visiting the Napoleonic fort. But you'd do better flying to Chopin, which is a pretty good airport – still an airport though!