Perhaps the "hard work" approach? For instance, a series of long corridors filled with traps (magical & mundane) which can ALL be avoided by laborious digging? (I had a PC overcome a magical tomb guardian in this way).
Perhaps the "Achille's Heel"? The guardian has a special weakness, known only to some higher level so-and-so, who offers to "give you a hint, for a favor"...
The "Cerberus Special"? Similar to the above, but the guardian can't be overcome in combat, but CAN be overcome through distraction and/or trickery, such as Cerberus was, by the offer of a cake.
The "Brains vs. Braun" Approach? The guardian is simple... An example is a Red Dragon who believes copper is worth more than gold, and is willing to trade all his gold & silver pieces for coppers (but would roast the PCs in a straight-up fight). The PCs must use methods besides combat to overcome the guardian.
The Riddle-Maze of Elder Wisdom - Riddles, traps, teleportation, ad nauseum. Many enter, but few are ever seen again! This is often boring and frustrating to the players (and GM).
The Keyed R-MoEW - All the riddles, traps, etc., have an answer relating to the same thing... For instance, if the key is "Wisdom", then when the PCs see the guardian, the solution may be to ask it to let you pass, instead of trying to fight it.
The Non-Sense Robot Trap - The guardian asks a bunch of questions to the adventurer preceeding the PCs, who looks up the answers in a book. After a time, it smashes the man (and his book) flat. When the PCs approach, the process begins, again. The PCs may think that these are riddles, and they need the "right" answer, but that is not the case... (The previous adventurer had them!) The truth of the matter is that the Iron Golem/Robot is looking for SENSE in the answers, and if it finds it, it will attack! Answering the questions with obvious non-sense will allow the PCs to pass, and get the treasure... On the way out, however, the guardian will ask them "Why have you taken these things?" If the PCs' answer makes sense, the guardian will attack, again!
