I find garters are very social creatures & really want the company of other garters. One I used to house alone escaped three times, each time I found her curled atop the nearest enclosure that housed another snake. In the wild, I rarely ever see a garter alone.
Garters are also very curious creatures, they love looking at contrasting bright colors & occasionally when my garters are restless and I know they aren't hungry, just putting a magazine cover against their wall for them to look at or laying the Sunday comics across the top of their is enough for them to stop "cruising for escape possibilities" and happily check out the pictures. They also absolutely love looking at themselves in a mirror. I keep a small mirror taped to the outside of my garter cages and often a group of snakes in a community enclosure will all be checking themselves out in the mirror together.