I think a Go REPL would be a very cool project. I think it is doable. I always have ipython up, when I program in Python.
There is even a C++ REPL somewhere. It basically compiles very time your enter the expression and runs the code. I think it can be done in Go (but someone correct me, if I am wrong here).
go run is surprisingly effective for playing around with code. I keep a main.go file in my home directory that I edit with vim and then run with ':!go run %'. Go compiles and runs fast enough that I almost don't notice its'a compiled language.
There is even a C++ REPL somewhere. It basically compiles very time your enter the expression and runs the code. I think it can be done in Go (but someone correct me, if I am wrong here).