Tower of Secrets

Hidden Disney references
Hidden Mickey Mouse

There is one hidden Mickey and Donald in the episode "Sinister Sculptor" as well as one Mickey and one Donald in "Some Day My Prints Will Come":

Hidden Fantasia references

There is one Fantasia reference in the episode "Knights of Gummadoon"

And there are references to Fantasia in "Light Makes Right"

Other Disney references

The "Tall Tales of Gummi Bears" book in the episode "Light Makes Right" also carries this page, mentioning the Nine Old Men and, specifically, Frank Thomas and Ollie Johnson (famous Disney animators):

Other real world references

That book in "Light Makes Right" has a lot more references than those I mentioned - there is a quote from Hamlet, a line from that pop song "I just call to say I love you", and some really wierd things. Here are all of the pages that appear in the book that aren't shown above:

"A-hunting we will go" has a Gummi version of Oscar statuette:

More to come!

I know I have missed a lot of other interesting real world references.