Making a whale:
We made a whale to sit inside about 5 - 6 years ago - the directions were found in Group's Vacation Bible Ship VBS kit. We just took two large pieces of black plastic (about 20' x 20', I think and taped them together around all four edges. An opening was left on one side (centered on that side) to accomodate one box fan, and we just duct taped the fan to the opening (so that it blows into the whale). On the opposite side, we cut a slit with scissors vertically (cut through both layers of plastic in a cut perpendicular to the edge - about 3' long).
When the fan is turned on, the whole thing inflates, and people can step through the mouth (vertical slit). For eyes, we put a black circle on the center of each of two white paper plates, and taped them to where it looks like they should go when the thing is inflated.
We were able to fit 8-10 people inside comfortably, and it was great fun!