Continuity mistake: During the scene where he is first at the cave, look in the sky. First, there aren't any clouds then there are in the next scene. If you keep watching, the clouds vary from the different camera angles.
Continuity mistake: When Tom Hanks is on the plane he sets down the pocket watch in the lower right corner of the groove in the seat, after an instant angle change it is closer to the upper left corner of the groove, with no time for it to move. (00:20:25)
Continuity mistake: At the Christmas meal, Chuck gets passed the candied yams. When he takes the yams, there is a spoon in the dish. When the shot changes to behind him, the spoon is not in his hand. (00:14:30)
Continuity mistake: When Tom Hanks sees the ship from the beach, it is obvious there is a pane in front of the camera. However, as he is jumping up and down through the scene, his shadow changes angles from in front of him, to a very low angle behind him.
Answer: No one knows what was in the package. I think Chuck doesn't open it in order to keep his sanity. The package is his only link to his life before being stranded, working for FedEx and making sure packages get delivered. He was determined to make sure it got delivered just as he was determined to survive and get back to civilization. Success in one meant success in the other. While Zemeckis has joked there was a waterproof satellite phone in it, the real answer from the script is, no joke, salsa verde. Its a care package with a note imploring Bettina's (the artist) husband (naked cowboy) to come back and spice up their life like the salsa.