:!sh will work, but it puts you in an entirely new context (environment and CWD). :sh is just plain awful and doesn't emulate anything correctly.

I recommended ^Z/fg/bg because it's the fastest, easiest way to get to a good, functional shell if you're not using a multiplexer (tmux/iTerm2). If you map ^Z in your shell to "fg\n" then switching is lightning fast.