Last week, we finally got around to something I've been intending to do for quite some time. We made terrariums (or vivariums): little plant/animal habitats. This is a fun project and, like so many of my favorite projects, can be made with household items and minimal craft supplies.
Materials for making your own toy terrarium:
cardboard box with lid (the hinged shoeboxes or mailing boxes are great)
thin, clear plastic (like bedding packaging, salad greens container, etc.)
magazines with nature photography
foamy filler (like polyester fiberfil, cotton batting, t-shirt or fabric scraps, etc.)
felt
cardboard scraps, plastic lid, cardboard tube
rocks, sticks
double-stick tape, glue stick, craft glue
X-acto knife
scissors

1) Begin by cutting, with your X-acto knife, a window in the lid and side(s) of your box, leaving enough room around your window to tape down the plastic. Cut your plastic next, larger than the windows. I used a clear package that held a mattress pad. I've had similar packaging from bed linens, storage bags, and other household items. You could also cut out from the lid or bottom of a grocery store-purchased spinach bin .
Use double-stick tape around your windows to tape down the plastic.
2) Cut nature photos from magazines and tape or glue to the interior of your box.
3) Line the bottom of your box with a shallow layer of filler. I used polyester fiberfil, but you could use any batting, fabric scraps, newspaper bunches, etc. Cut a piece of felt a little larger than the base of your box. Line the edges of the base with craft glue and press down the edges of the felt to cover the bottom.
4) Add your plant life. I glued some nature images to thin cardboard (eggs, leaf) and cut them out. I also glued felt to cardboard and cut out grass/plant and nest shapes. For our water basin, I glued felt around a 1/2" deep jar lid, and filled the inside like I did the box-with fiberfil and felt to cover. Cover a cardboard tube with felt to make a log. Add natural elements like rocks and sticks.
5) Finally, add your animals. We used little toy frogs, lizards, and snakes, that tend to come home in party bags.
We did not decorate the box exterior, but you could take it a step further and do that, as well.
My kids call it the frog house and are asking for another, since this one is rather small. I did construct this one by myself, primarily, but I'm thinking next time they could choose images, glue them on, make some plants, add the nature elements, and decorate the box exterior. Older children could participate more fully in the design and construction.