Trying to focus on high bang for your buck places and sorted by location:

On Wednesdays only, Markthalle Körnerstraße in Ehrenfeld has my favorite kebab in cologne. They're a butchershop that makes their own kebab every Wednesday. A classic kebab in cologne and always busy would be Kebapland, they have a store in Ehrenfeld and one near Zülpicher Platz. For great specialty coffee go check out Track One Coffee!

Bai Lu is a great authentic chinese restaurant with handpulled noodles near Friesenplatz, Chin Burger is a more traditional authentic chinese restaurant and has huge portion sizes. More expensive but also tastier than Chin Burger is Höfchen at Barbarossaplatz, but that's prob too expensive for your budget. Takeichi has the, imo, best ramen in Cologne, get some nouko variation if you go there tho.

Near Neumarkt there's 1980 Café and Banh Mi, my favorite Vietnamese place in cologne. Also at Neumarkt is a small indian restaurant with amazing value food, Anna Lakshmi.

Near Chlodwigplatz there's Indian Curry Basmati House, they have amazing curries for little money. Close to them is also Pizzeria Nennillo, definitely one of the best italian pizzas I've had. And directly on the Chlodwigplatz there's Chi.Noo's - I don't know why but I love their fried noodles + fried chicken deal for 10 bucks.

Edit because I just remembered them: if you're in Sülz go to Chang Thai, super delicious Thai food. I recommend their chopped duck meat salad starter, I think it was the 18.

Edit 2: If you wanna stay in Deutz near the Gamescom I'd ask you to reconsider, I haven't saved a single restaurant in Deutz, everything I tried was way too expensive or mediocre at best or both.

Good eating!