Early Park Access is to Harry Potter lands in both parks so use EPA to hit whatever park has it for the day and as
@Mad Dog says, explore! If you have some extra spending cash you can get a wand for your kid (and yourself! I'm 28 and have 2) and perform some spells around each of the Harry Potter lands.
While you're at Universal Studios, you can ask for the "behind the scenes" tour for MiB and the Mummy, for MiB you get to meet the aliens in the control center and for the Mummy you get to go under the track! Usually middle of the day is your best bet for them having enough team members to take you but it's not guaranteed. (For MiB you ask RIGHT when you get off the ride)
Water rides in IoA are a blast, we always hit them right before we're planning to go back to the hotel so we're not soaked the rest of the day.
You can walk across the street to Sapphire Falls resort and take the boats into city walk isntead of taking the bus (when I stay at Cabana I ALWAYS do this). They ask that you use the river walk bridge to cross the street but if you're feeling adventurous / make sure your kid is holding your hand you can just walk right across.
I don't know if your express gets you express in Volcano Bay but they do have Express there and it's once per slide (vs. unlimited in the dry parks). Highly recommend it, especially on the weekends. Don't use it until later in the day though, if you go when the park opens you can easily hit every slide with close to no waits!
I'm leaving work now but if I think of anything else I'll let you know!