Don,

april and may might be a little early for airshows in Europe. May is the month that may have a few but most of it starts in June.

Depending on what kind of aircraft you like you might want to visit La Ferté Alais in France which has a respectable amount of historical aircraft which they pull out during their show. Most of the time this show is around White Monday (23 May 2010 next year).
Duxford may have an early show in May but I am not sure.

For museums, there are many interesting around like Duxford, Gatow (Berlin), Soesterberg, Brussels.

André