I would suggest get there first thing in the morning and enjoy the park. You should have plenty accomplished by the afternoon, especially if you get there at opening time, or if you stay on property you get EE, which is even better.
From there, you can determined if you're done for the day and move onto going back to the hotel to rest for the evening (since tomorrow is another day, as you said this would be a long weekend), do something else in Orlando in mid-afternoon, or to head over to IoA if you have the two-park tickets, as
@Lynne G suggested.
Or even if you want to relax a little bit, but want to stay in the area, perhaps enjoy a full service meal and take your time to relax in the restaurant before slowly heading to IoA, or even enjoy a movie at the City Walk.
Also, another thing to note is that Sunday is typically slower not only generally for the parks, but HHN as well, since people typically have to get to work, or school the next day.
However, as a whole, I do not believe HHN affects the crowds really, other than the time in which it starts to transition from regular park hours, to HHN hours.