Say one is bored on a Saturday afternoon. Is it permissible--if one finds it entertaining--to pack his suit case when leaving on a long trip the following day?
There is a general Rabbinic prohibition, called "Hachana"1, against preparing on Shabbat for activities after Shabbat. Packing on Shabbat for a trip after Shabbat would seem to fall squarely within this prohibition.
1. See the linked Halachipedia article for lots of detail and sources.