The Illuminations cruise is really nice, and makes a great surprise. The best part is that you'll have front-row seats for it, and you wont have to wait for them or deal with pushy people! We really enjoyed ours -- my DD was 3 1/2 years old back then, and she had a great time, too.
Unless you've booked a suite, you'll be in the Hawaii building, not Tonga. And no, there are no parking lots anywhere near Hawaii. To me, it looked like most of the GVCL rooms were overlooking the quiet pool & gardens -- quite nice, actually, and a much better deal than paying for LVCL!
Having the snacks in the lounge is great, plus having the CMs there to help you make last minute dining reservations & answer any questions you might have is very helpful. Also, there's a fantastic view of the castle, fireworks, and EWP from the lounge! I can't tell you what room to request to get a better GV, but I'm sure some of the other posters on this board can help you with that.