@bjs I like the field idea. Affordable ion probes that don't require re-agents would be ideal for our Costa Rica stuff. We really don't want to take "chemicals" by plane/customs, and try to avoid expensive electronics if possible. We've used "garden" pH meters in the past - they're more likely to break, but it is no loss when they do.
For air/water temperature measurements, we/the ecologists use "HOBOs": http://www.onsetcomp.com/
More ecology/hydrology than geology as you deploy them and they record the temperature periodically over days/months. Simple micro-controller and penny battery, and the price is right (I think less than $50 each, and they're the kind of thing that can be bought a few at a time). A "RasPi HOBO" as it we, could do a lot more - e.g. support simple telemetry, multiple sensors, etc.
With my geological background and the fact we're about 7 miles from Volcan Arenal, I keep trying to think of geological things we can do, and there's not much. Deep soils, lots of clays. There's a pre-European ash layer that outcrops on the study property but in only one location.
If you think a magnetometer could be cheaply built, then that is one possibility, but I suspect we'll only pick up water pipes. Yes I'm sure the property is underlain by dykes, but then virtually the entire country is igneous
The geophysical technique the property is really crying out for is a Bouger inversion to measure the density of the hills. We could drag a physics major along, and do a really neat experiment due to the large elevation changes. Unfortunately a gravimeter is way out of the budget of a small science department.
The other affordable technique (and probably within reach of your RasPi system / our budget) would be a 4 probe resistance meter. We could take that and do some Schlumberger/Wiener networks, but the problem then becomes one of interpretation. I've got a nice wiggle on the graph, but what does it MEAN?
Sorry, I'm digressing.