Unless it's just one of you travelling, I would definitely recommend these guys. 80PLN for a shared option from Warsaw Modlin Airport to Warsaw old town / 110PLN for a private transfer (versus a bus at 35PLN per person + 15 mins walk and waiting around for bus to arrive etc). Friendly service and can be booked online via email with quick response (within 24hours)

One of the most underrated places in Warsaw, I always make a point of visiting when I get off the train as it is right near Centralna station, it's cheap and only takes 20mins to visit. This is the Warsaw I love - the secretive and nostalgic Warsaw inhabited by the Fotoplastikon. A visit is basically time travel to the interwar period. I can't understand why more people - even Varsovians - don't know about it. Highly recommended.