It depends on a lot of things... How tired you will be. How old your kids are, how hungry you will be, what resort you are at, which tickets you have....
We just got back. We flew out at 7:30 and arrived at MCO at 11:30, by the time we debarked, got our baggage, met our town car, did our grocery stop, checked into the hotel (BWV) and dropped our bags it was 2:00. We'd had nothing since Starbucks scones and granola bars since the morning. The kids had a peanut butter sandwich in the room and we decided to go into Epcot to the Biergarten for lunch, then did a little of Future World, and turned in early. But my kids don't nap (ages 4 and 5) and hadn't discovered the pool (yet) and we had LOS passes - so we weren't burning a day. And the kids wanted to get on rides, so they were pretty hyped to head into a park.
I have decided not to push myself to do any evening events (Illuminations, Specto, Fantasmic) the night we arrive. Always disappointing because we are too tired - we seldom make it that late (with our non-napping kids) even when we didn't get up at 4:30 to catch a plane.
Some thoughts:
Late lunch or dinner at your resort, another resort, or a theme park. A lot of people want Chef Mickey's their first night. I myself wouldn't bother unless we were staying at a MK resort.
An afternoon by the pool.
Checking out your resort. Many of the resort areas have plenty of nooks and crannies to spend an afternoon in.
A nap.
An afternoon at the park. Some people can't wait and can go until midnight. Others only want to nap. You know your party better than we do.