Replace electric water heater with gas tankless (electrical connection)
In the panel, move the white wire to the neutral bar, and move the black to a 20A breaker.   Fit a 15A or 20A duplex receptacle.  And you're done.  

You can use up to about 19.5 amps of power off that receptacle, since the water heater won't take much.  

If you want two full circuits good for a total of 30A@120V, in the main panel, leave the black wire on the 30A breaker, still move white to neutral. Then fit a subpanel, of any size.  Black to a hot pole, white to the neutral bar, ground to a different ground bar that is isolated from neutral.  Only every other row of breaker spaces will work. Fit two 15A or 20A breakers and use each one for a different circuit.   If you place the breakers in the wrong row, don't bust out more knockouts, switch the black wire to the other hot lug.