Changes in pH when adding substances other than acids or bases
You could use an alcohol or acetone but they are slightly acidic (though much less so than acetic acid; also alcohols react to form esters). Now effects from acidic nature can't completely avoided as even a molecule as non-acidic as methane still has a pka of 50. I think an aliphatic ether such as tetrahydrofuran (THF) or diethylether would be the best substance to add as the ethers are only very slightly basic and are miscible with water.

More simply:


  what substance could I add to change the pH considerably?


Why does it have to be a substance? The $K_\mathrm w = 10^{14}$ at $\pu{25^\circ C}$. If you add heat the $K_\mathrm w$ will increase and thus the pH will increase.