I had a Kanken when I had front row seats for Centre Court. I took the same Kanken last year when I was up in the gods. It needed to be big enough for a shawl (protecting against the sun or for warmth in the shade), a jacket for the inevitable rain, a water bottle, sunscreen, snacks etc.
Comfy shoes are also vital a there is a lot of queuing and going up and down stairs.
Obviously if you're in the Royal Box that will be different as you don't get to see any of the tennis anyway as you're whisked away for afternoon tea.