Sorry for the late warning. Dutch Spotters are hosting another Rotterdam Aviation Fair inside the terminal building on Saturday 18 April.
Parking and entrance is free. Inside you’ll find many booths and tables selling all kinds of books, DVDs, models, slides etc.
You can also take part in ramp tours of Rotterdam Airport (I did one of these a couple of years ago and it proved very fruitful!). There’s also the chance to take a sightseeing flight to Amsterdam.
The event is open from 10am to 4pm. See this link for more information.
Upstairs, Rotterdam has a great viewing gallery overlooking the apron.