Stamps are great for various art activities. Most of the stamps you will find will not be very valuable and they are lovely to create pictures with. Many people have been doing this since Victorian times. Here are some examples of modern art with stamps:
ArtStamped - fantastic collages using stamps - not only giant pandas, check out the website.
Signature Stamps - great artistic ways to display stamps or even wear them!
Tonia Jillings - beautiful art inspired by and including postage stamps - you can hardly tell where the stamp ends and the art continues.
There are also many more excellent artists using stamps, too many for me to list. Many display their work on Instagram.
There are many ways to learn from stamps and there are some brilliant activities on many other sites. Here are some examples:
Exploring Stamps YouTube channel - excellent videos on many different topics.
Stamp Active - Stamp Active supports StampIT but also has many other activities.
The Postal Museum - a museum of postal and communication history with some great resources.
The Digital Philatelist - a great networking site to find out about many other stamp websites.