We're doing this decal (well...not the actual decal, my sister is going to paint something similar on the wall)
If you go to roomzaar.com and search for jungle or safari under the nursery section there are some great ideas there.
We did a blue & sage green safari theme.
We painted the walls blue with light blue stencils. I trimmed the lampshade with blue and green ribbons. I made the valance and bed skirt out of silk shantung. I got a blue breathable bumper and I bought a bunch of brightly colored animal themed sheets from different bedding sets. We are not moving the crib into the nursery until the guests leave (and we can move out the bed) but you can get a sense of the theme from these photos:
Here is the bedskirt:
The giraffe will end up in the corner where the bed is now:
Our tree isn't a decal but a 3-D tree we made from fabric, foam, a wooden base, and a contruction tube used for pouring concrete. It is in a girl's room, but would work well for either gender. There are tree holes with jungle animals (sloth and coati), and monkeys hang from branches and frogs perch on top of them. This pic doesn't do it justice, but it will give you the general idea. The vine we made from a cotton batting "tube" used for upholstery or something ... we just dyes it green and added the leaves. The fabric leaves on the ceiling are all different shades of green and textures/sheens of fabric.