I was thinking of something like using poisons or something more creative like quickly having every soldier dig small holes into the ground which would trip approaching cavalry.
In movies like Last Samurai, we get a good example of the kind of ingenuity I am talking about.
Medieval warfare did feature quite a bit of creative tactical thinking, including night raids, ambushes, raids, and field entrenchments that could be as simple as a ditch dug into the ground or a larger fortification involving earthworks and sharpened stakes. It's been a long time since I've seen Last Samurai, but in general the sort of traps Hollywood movies like to throw into medieval battle scenes are ridiculously elaborate. They're designed to create fun visuals, not for practical battlefield results. As for poisoning an entire army, I won't say that it never happened, but no examples from the time period I study immediately spring to mind. It would be logistically difficult to poison an entire army composed of thousands of men and horses, requiring a fairly large amount of poison and an agent with speed that would make the Flash blush.
Read Anna Komnene's Alexiad (regarding events occurring in Byzantium from c. AD 1075 - 1118) and you'll find plenty of ingenious and wholly devious tactics used by Emperor Alexios and his contemporaries.
Some examples of ingenuity:
Using deadly rolling logs deployed from cranes on city walls to break up enemy troop formations.
Using a small assault team armed with Greek Fire grenades to burn down enemy siege equipment.
Using amphibious night raids to demoralize encroaching enemies, forcing them to abandon their ambitions.
Specifically capturing an enemy general's regalia and parading it around to break the opposing army's morale.
Leaving an empty camp for an enemy army to capture, then storming it by encirclement.
Using knowledge of a solar eclipse to trick the enemy into thinking that the army commands divine favor.