godoc -http=:6060 -play -index
Then navigate to localhost:6060 in your browser.
misc/goplay is the answer for offline play.
Interesting. I could have sworn that I've used that completely offline. Live and learn.
| $ cd $GOROOT/misc/goplay |
| $ go run goplay.go |
| load http://localhost:3999/ in a web browser. Works perfectly for me. |