What are the best things to eat/drink for adequate nutrition if one is not able to eat solid food?

Depends on Reason: It really depends on the reason you are unable to eat solid foods. If from a stomach bug, you want to consume what you can tolerate and what is appealing with an emphasis on hydration (alternating water and gatorade). If it is because of, say, a broken jaw, protein shakes are a good bet. You can also puree fruits, vegetables, and protein so that you have relatively normal nutrition. Good luck!