What's the easiest/fastest way to experiment with bitcoin-cli?
Yes, you can run bitcoind with the -testnet option.  This starts a separate process that runs independently of the mainnet bitcoind.  You can control it with bitcoin-cli -testnet.

Note that you will have to download and verify the testnet blockchain before being able to do very much.  This also takes some time / bandwidth / disk space / CPU, though much less than for mainnet.  Depending on the speed of your CPU and Internet connection, it might also make the mainnet download take longer if they have to share resources.