Whenever you go anywhere that is different than where you usually are...grab yourself some post cards to send to your missionary. If anyone else is going to be there that knows your missionary...grab some for them as well. This past summer Jeff and I went on a business trip to San Francisco for a KFC convention. My friends and family were great sports to write post cards to Matt.
Missionaries LOVE mail! They don't care who it's from or the size or shape. They just want to know that they're not forgotten and what's going on at home. Make it easy on yourself and others by bringing labels (return and for your missionary), and post card stamps with you on your trip. If you purchase the post cards and have them already pre-stamped and addressed, it is much easier for someone to just jot a few lines and hand it back to you and you can pop it in the mail. Most hotels have a gift shop that sells post cards and they also have daily mail pickup. Just ask at the front desk. Some hotels even have free post cards of the hotel.