Try drinking chilled Pedialyte or similar drink, they're intended to rehydrate well. Yes, they're marketed for kids, but damn if they didn't work like a charm for me last time I had a stomach virus. Goal is not just water replacement but maintaining a good electrolyte balance too. Although It probably won't happen, it'd be bad to get too-low sodium or potassium in addition to the flu.
Tylenol to keep the fever down. No alcohol, that'll just dehydrate you worse and can cause liver problems if you're taking tylenol.
Eat what you can handle, but avoid fatty or spicy foods.
If the cough gets painful, try Ricola original formula cough drops. They taste weird at first but you get used to them, and they work (at least for me) much better than Halls. Let them dissolve, don't crunch them like candy.
Finally, if it persists for more than a few more days, see a doc, it might not just be flu. Edit: also, if you really can't keep yourself hydrated, you might want to see a doc. Dehydration can be dangerous, and if they really, really need to, they can keep you hydrated through your arms instead of your mouth.