Where practical and safe (and unlikely to cause damage) the ability to go inside larger aircraft and/or steps up to the cockpit to be able to look inside.
I appreciate that the steps and other stuff needed for such does detract if one is trying to take photos or get a wider view, however.
Nonetheless, it is good to get up close when possible to do so.  
Also, I find that my wife (who is not into aircraft) gets bored just looking at outsides, but lasts a lot longer when there is more to see and read and when she can go up and/or into some exhibits.