Places to stick stuff:

In front of the glovebox (that's if you don't have an early model with
dealer installed AC there.)

Under the rear seat. Lots of space, but oftentimes used for tools.)
Under the front seats. In a Vanagon without swivel seats, this is
relatively easy access. Just slide the seats forward and it is right
there.

After that, you have to start getting creative. The side panel on the
drivers side of the vehicle. If you remove the panel, there is a fair
amount of dead space between the outer skin of the vehicle and the
interior panel.

If you have an overhead AC unit with the evaporator in the back (and
duck work that goes forward) you also have the possibility of putting
stuff behind the passenger side D pillar (which is part of the support
for the evaporator). The drivers side has the tubes, and electrical
connections for the fans, but the passenger side is more or less empty.

That's about it for the insides. Then you can start thinking about
outside locations. lots of places there...all exposed to the elements.