Sugru is great for window decorations. It clings to the glass but can be easily removed later.
Things like spider webs are surprisingly easy to make.
For this window decoration, I used:
1 1/2 packets of white Sugru (a certain kid decided to bite some of my Sugru packets, and the tiny holes cured half of what was in them)
less than 1 packet of black Sugru, preferably uncured (but I used cured stuff... see above)
scissors if your black Sugru is already cured (in case you have naughty kids at home too)
Step 1: Prepare the Sugru
Put a drop of soap into a container, then fill it with water. You'll want to dip your fingers in this to keep them from sticking to the Sugru. Make sure your hands are wet before you take the Sugru out of the packets.
Open the white Sugru packets (it's easiest to use scissors) and knead the Sugru with wet fingertips. Divide the white Sugru in half. Set one half aside and divide the other half into several smaller sections.
Don't open the black Sugru until the white part of the web is complete.