I love tuna for lunch!
The Weight Watchers brand are the perfect size for one person. I just mix it with a teaspoon mayo (you can always use lowfat mayo, but I'm kind of anti-lowfat... I think you're much better with full fat and portion control), thyme, a bit of minced onion, and pepper.
Also eat a lot of caprese- one ball of mozzarella, one tomato, some olive oil, salt, pepper and oregano and you're golden!
I'd love other ideas too as I'm on my own for lunch most days and I'm feeling pretty uninspired lately!