There is a cafe next to the ticket office where you can get drink and quick food. since the journey from Aswan to Abu Simbel is long (3 hours x 2 ways + 2 hours on site), you should bring along water and snack to be consumed on the bus. If not, go to the cafe.
There is a small cafe at Abu Simbel Temple but the prices are expensive and there's not much choice. I would recommend you take some snacks with you on the bus trip and take some water for while at the temple. Our trip took around 8 hours in total (3 hours each way in the bus) and we were glad we had something to eat.