My DD is 6, and in 1st grade. We have long days, too - school from 8:30 to 3:30, and gymnastics from 4:00-6:30 M-W-TH. And on W&TH, she has to come with me to church choir practice right after gym. Those are the toughest days.
I make sure to have a fun and healthy snack ready for DD when I pick her up from school. We have to head straight to the gym, so I let her eat and tell me about her day during the drive. Occasionally I will get her a hamburger and Coke from McDonald's (not healthy, but her favorite). That always puts her in a good mood.
I let her have as much downtime as possible at home, since so much of her day is so structured. That helps, too.
Last year, she had gym from 5:00 to 6:30 twice a week, so we had time to go home for a while after school. I actually like this year's schedule better, though.