At Google I/O 2014, an economical introduction to "VR" (Virtual Reality) was released in the form of a folded cardboard assembly to hold your smartphone running special applications with an image for each eye. Named "Google Cardboard" for the primary material, outside the conference (where it was given away) buying it from third-parties could cost over $20. While I did get the original "Cardboard", my Nexus 6 was too big for it, the construction was complex, and the input mechanism (a magnet that interrupted the phone's internal compass) didn't work with every model.
Google I/O 2015 released an improved model of a more simple assembly, adapted to fit larger phones. The actuation mechanism was changed to a flap that simulated a finger touching the screen, making it compatible with more phones (at this point there are Cardboard apps for the iPhone and of course the Android platform, but not Microsoft's Windows phones). Instead of purchasing the reintroduced version, I bought a Mattel View Master "Starter Pack" that wasn't much more ($29.9x for almost every retailer out there).
While I find the plastic and rubber construction of the View-Master much more sturdy and comfortable, the "Starter Pack" applications want to do huge downloads with little functionality until you make more "in-app" purchases. Running the stock Cardboard demonstration apps works well, and there are several short apps for free in the Google Play Store. Be warned that VR applications drain the phone battery very quickly and generate heat from using the phone's CPU so intensively. In fact, the back of my phone became discolored from using the apps [Addendum: the phone was stained when I put it back in my carry case while it was still warm, and I was able to clean it later], although the phone was never more than being warm to the touch.
There is not a strap to hold the View-Master to your head, although viewing sessions are likely to be 20 to 30 minutes at most with the battery use and heat. The Nexus 6 (with close to a 6" screen) is the largest phone that the View-Master holds, of course it needs to be removed from any third-party case before being inserted into the goggles. The concept isn't advertised for young children (specifically for the View-Master: age '7+'), but my almost 4-year-old Granddaughter loves to watch so much it can be a difficult time to get her to let go.
Get the Mattel View-Master for the goggles, not the more expensive applications (although it is certainly expandable if you get further into the Google Cardboard VR experience). I expect more Cardboard apps to appear with time, and phones to improve for CPU use and heat. At under $30 with free applications able to be downloaded, it is a low-cost method to experiment with virtual reality for now.