Here is a fun Ghostbusters craft we recently did. We printed the Slimer paper cubee from Cyberdrone's deviantART page. He was a little too tricky for a six year old to put together so I did most of Slimer but my son had a good time making the tomb and enjoys playing with his Ghostbusters paper toy. You may also want to stuff Slimer with torn up cotton balls or polyfill for added stability if he will be more than a decoration.
For the tomb get a small box. You can cut off one end if you want for a spot to put Slimer in when he's resting. Cut a piece of construction paper the same size as one end of the box but give a little extra room at the top for the arch effect of the headstone. You can then write "Tomb of Slimer" or something similar on it and decorate it to give it an old, weathered look. Glue it on and Slimer is all set!