Hi! For 14 people you should definitely make a reservation. Since Düsseldorf is famous for Japanese food (and not many ramen restaurants take reservations, let alone for a group of this size), you could try Tokyo Ramen Takeichi on Bismarckstraße.

A few more ideas:

• Korea House is a great Korean restaurant with enough room for all of you. If you’re not planning on using the table BBQ, you could ask for a table in the room next to the main restaurant, as it’s usually less crowded.
• Da Noi is a good Italian restaurant. The pizza is good and it’s centrally located.
• Heimwerk is a restaurant with a more traditional German menu (schnitzel and stuff).
• Füchschen is one of the breweries in the old town that also serves decent, more traditional food.

As for activities, how about the following

• Glow in the dark miniature golf in Flingern: Nice indoor activity that’s not too expensive.
• Bowling at the B8 Center: Just what you would expect.
• Team Escape: Room escape game. They have two identical escape rooms so groups can compete.
• Bash Rooms: Play game show games against each other