I got these pretty semi see through envelopes a while back at a paper store sale...I cut magazine clippings/fun papers to twice the size of the cards then folded them in half around the cards and slipped them into the envelopes to hide my letters and customize the envelopes. Super easy and pretty!
Do you send letters to friends or just stick to email/facebook? I'm going to make it a goal to send one letter to a list of friends/family I don't see much before the beginning of summer...we'll see how that goes! If you have any questions about how I did the cards pictured here just post in the comments!