This spot seems to have gotten bigger since I last went (which was around 97 or so). It's all about planes here. The coolest part is that they are not all roped off and you can roam around them to get all sorts of cool angles and details. They even have benches scattered around. There is also an indoor building with other cool things to sketch.
They've got just about any kind of flying vehicle you could want. Helicopters, giant troop planes etc.
It's right off the 215 in Riverside CA. Here's their website: http://www.marchfield.org/indexp1yourvisit.html
Map here: http://www.marchfield.org/map.html You can spend all day here.
Map here: http://www.marchfield.org/map.html You can spend all day here.