Listen to music, watch movies, looking out the window.. Talk to people on my left or right. Most of the time, I'd just drink, chow some food, or walk around to stretch myself. Sometimes, I'd ask FAs to mix me something like Coke/Sprite/Fanta mix or Combined Juice drink...

If I'm dealing with extra long flights, I'd probably try to sleep half the flying time or when the crew isn't busy, I'd like to chat with them..