Early entry will get you into the parks an hour before they open.
Toontown opens an hour after the parks open, so you can go into Toontown right when Disneyland opens, instead of waiting the hour.
Your EE voucher is only good for one day, irrespective of going to Toontown. However, the Disneyland Resort hotels have been running a special where their guests can take advantage of EE each day that they are staying at the hotel. (Nice perk)