Okay, technically you no longer need to send an actual invitation to the President- they have a handy form online that you can fill out for a wide variety of milestones- baby’s birth, major birthdays, Eagle Scout/Girl Scout Gold awards, etc. You can find it here. Note, this service is available to US citizens only.
For a response to Mickey and Minnie Mouse, send it to:
Letters to Mickey Mouse
P.O. Box 10040
Lake Buena Vista, FL 32830-00100
If you are a British citizen (or Canadian, Australian, or New Zealander), the Queen will send you a greeting for a milestone birthday (and they mean serious milestones- 100+ only) or wedding anniversary (starting at 65 years!) but (not for a wedding). Find the form here.
And if you have a deceased loved one who is a Veteran of the United States, you can find information about requesting a flag here.