Some good thinking here, so I will just add that you could use smartphone even without worrying about using it as phone. My old android phones work fine as mini tablets without cellular service. Given that, something like my old galaxy s3 would probably available used for cheap. Battery life is often a problem for an older phone like that but if you only need it to last ~3 hours, maybe it doesn't matter. I do recall that the s3 also had barometric capabilities.