Hey, u totally should have a trip to europe or something, go somewhere unknown where you don't know anyone, somewhere where you can act like a 5 year old without any worries.
How do you keep a youthful zest? I think by being willing to be exactly who you are, and never apologizing for it. This way you can avoid the habits that others will try to get you to adopt to help you fit into the role of "adult" more seamlessly. You can be serious, but also you can be capricious, whimsical or downright ridiculous if you feel the need. :)