I was in the same boat a while back. Here are some suggestions that actually worked for me.
1) Change your lunch. Yep, that's right. If you're eating high glycemic foods at lunch the insulin spike will drop your blood sugar in the afternoon and early evening when you leave, and you won't want to do anything because you're lethargic. Don't eat pastas, rice, sugars for lunch. This change had the most effect on me.
2) "Work out" at work. Yes, you can do it, just probably not weight lifting. Take 20+ minutes, probably before lunch, and briskly walk around the building a couple times. Is the weather bad? Then go up and down the stairs for 20+ minutes. This isn't a hardcore workout, but it keeps your metabolism up just enough so you're not lethargic.
3) Pack your gym clothes and go straight to the gym after work. Don't even stop at home. Sure, the gym is more crowded right after work, but you can deal with that. Or leave work late and go there after the crowd is gone.
4) If your diet is in order, you need to make your meals/snacks in advance and take them to work. I find I can make a 3 pound meat loaf on Sunday and it provides afternoon meals for about a whole week.