A lot of things. It's almost sad.
Any bird of prey, especially that hawk sound. Basically any animal they morphed, McDonald's, people with the respective names, groups of 5-6 animals together, the topic of aliens, the topics of slavery/morality under pressure/etc... The list goes on.
Also, when I look at the sky and wish I could fly, usually I'll also think, "Damn. If only I could morph."
Thinking about Animorphs is a lot like the game for me. It's that annoyingly ingrained into my thoughts, but I guess that's because it helped me a lot when I was younger.
By the way, when you read the above statement, you lost the game too.