Curiosity, desire to learn about everything.
Fearlessness, especially when it comes to being social. My three year old runs up to kids she's never met before and instantly likes them and plays with them. I wish adults did that.
Sort of related to the above comment - not judging people based on appearance or other superficial things.
Excitableness (I don't know if that's a word actually), being thrilled to do even the simplest things because they're just fun and life is all about fun.
Not being self-conscious. Happy with who they are because society hasn't told them who they shouldn't be yet.
Imagination and creativity, the ability to look at a stick and get enjoyment out of pretending it's a horse, or making up lives for their dolls.
Watching my daughter is actually very inspirational. She teaches me to enjoy and be thankful for the little things. She teaches me to let go and have fun when I might not otherwise. She exudes love and acceptance.