As I mentioned, one of the reasons I picked up the violin was because it was easier to travel with than a guitar (not that I haven't found some nice travel guitars, but that's another story). It fits in an airplane overhead and is easy to carry!
However, I want to be considerate of the folks in the rooms around me when I'm playing in my hotel room. I decided I needed a mute -- there are a lot of gizmos you can attach easily (and not so easily) to your violin to reduce the sound. I wanted something simple and effective -- it had to be easy to slip on and off and it had to work!
Turns out it was really easy to find, too! Head to your laundry basket and grab a spring clothespin. Okay, go ahead and grab two -- just in case. Slip the first clothespin onto the treble side of your violin bridge. Angle the pin downwards so it won't interfere with your bow.
That's it! Give it a try and you'll be amazed at how the sound is muted. If you want even more muting, put the second pin on the bass side of the bridge. (When you do this, it looks like your violin is wearing a mustache!)
Doesn't get much easier than this -- plus there's probably room in your case for the two clothespins. Simple, cheap, and effective (although it does look a little goofy).