Dining is informal. Passengers usually eat the same meals as the officers. After dinner it's either books, bridge, or backgammon, or talking to other passengers or perhaps a movie. In port, shore plans are made independently by yourself or with fellow passengers. Port stops may be for a couple of hours or for several days.
Some lines allow passengers to join for one segment of the trip instead of the whole voyage.
Some allow one-way passage. Reservations need to be made as far in advance as possible because of limited cabin space available. Cargo ships go where the cargo is, so always confirm dates of sailings and itineraries because schedules can change. But modern containerships have relatively fast and predictable schedules. Sail-fly and layover options are often available.