Pick a Card, Any card - Looking at the AD&D 2E Deck of Encounters
Probably the most useful thing ever to come out of AD&D 2E was NOT the endless collection of Complete Player Handbooks (hard to believe, I know). Nope, the most useful accessory I found in my nearly complete collection of 2e books and assorted crap is my Deck of Encounters Sets One and Two Boxed sets.
These little treasure chests offer nearly limitless opportunities to challenge your players and work very well in a sandbox style campaign.
Generally, I'd grab a dozen or so cards before the party would head out to their adventuring destination (if they had one) and randomly pick a card when circumstances dictated. Most were level appropriate, one or two would be weaker, one or two would be on the more dangerous end.
It was a great tool to lower prep time and still keep things exciting and different for the players. They have been a great rediscovery as I've been doing renovations. I'm sure to have more to say about them as I spend time perusing them again ;)
The Minotaur for Old-School Essentials
*Requirements:* Minimum STR 9, Minimum CON 9
*Prime Requisite:* STR and CON
*Hit Dice:* 1d10
*Maximum Level:* 8
*Armor:* Leather, including shield...