A language dictionary--say, a French dictionary, for use in translating a word or phrase, needed to go to the next stage
Similarly, any reasonably small reference work. Asking them to bring an entire Encyclopedia Britannica would be a bit excessive....
A photo of the area, posted on the cache page. You've used Photoshop to alter part of the photo, as a clue. (This idea was mentioned here some time ago)
Binoculars/monoculars, to stand in a particular spot and, looking in a particular direction, spot a clue/object
A ruler/tape measure, to measure the dimensions of an object, giving them part of the coordinates to the next stage
A cell phone, which they'd use to dial a number provided at one stage; a prerecorded message would give them instructions for proceeding to the next stage. Obviously you'd need an idle phone line for this
There must be many other possibilities....