Line one or more students outside the series of masking tape lines. This should be oriented appropriately (North in your room should also be North).
Explain that this is the equator. They should step to the first line, that's 10 degrees, the next is 20, the next is 30. Our school is at approximately N34 degrees (teachers adapt to your own coordinates).
Have another student or students start the other direction- this is the Prime Meridian. They stand outside the first block. The first line is 10 degrees, then 20, etc all the way to between 80 and 90, we are about 86 degrees (written -086W).
The students should intersect. They found an actual point.
Show the students a GPS. Pre-program the coordinates of the hidden container. Tell the students they are going to hunt for the treasure.
Now outside, show the compass arrow on the GPS unit so they can walk to the treasure. You might want to hide several items and give each group a different set of coordinates, if you have more than one GPS unit, if not each group will wait for their turn.
Each group should find their item and then chose a spot to hide a cache for the others to find. Students should walk with the GPS unit on, to observe the rapidly changing latitude and longitude. When they have selected a spot (ideally away from buildings and metal to avoid interference), they should record the coordinates. A GPS unit should be allowed to "settle out" to gain reliable coordinates. Most GPS units will take many readings, so give it time.
Check the coordinates again by entering the coordinates on the GPS and see how close you will get to the selected spot. Students enjoy trying again and again to lead their classmates to their selected spot.
Do some problem solving to figure out why the coordinates might be off (metal, atmospheric interference, etc.). Get lots of feedback from the students, they really will enjoy this high tech hunt.
Return to the classroom to grid and place markers (with degrees and labels for the prime meridian, equator, tropic of Capricorn, tropic of Cancer, international date line.) Label the lines of latitude and longitude on maps and globes.