Having certain materials always prepped means never having to be at a loss for "stuff" just because players went in an unexpected direction.
1) Ye olde list of generic stat blocks. Typical beasts, NPC types, and a few genericus maximus monster types work fine.
2) Ye olde list of names. For characters, small towns, taverns, ships, books, etc. There are lots of generators for these out there and building some stock lists is a snap.
3) Ye olde list of plots/motivations. Some general things going on with the principals involved left blank. Example: X needs money to fund something. It is a secret and he/she is stealing from Y to get the money. A good mix of good guys, bad guys, & greedy guys that want something and are attempting X to get it.
4) Ye olde stack of generic maps. Caves, small dungeons, villages, forts, taverns & other typical locations likely to come up.
Once you are well stocked with these items, there is very little that can't be cobbled together to get through a session sans any other prep. The only other thing needed is scratch paper to jot down what elements you used to maintain consistency. So if the players decide to suddenly visit the next village to see if they are experiencing the same problems as the one village you have lovingly detailed (AND THEY WILL!!), using your store of material you can come up with a named village, a map, a named tavern, and various named NPCs (with stats ready if needed) that the characters can talk to and it will seem like you had the place designed already.
Never throw anything out. Always recycle and keep building your store of generic material. The more you save and organize this stuff the easier campaign prep gets.