Horizon 

The exact values depend, or course, on the local topography, but we usually get close enough simply by assuming the Earth is a sphere. We need to know the size of the Earth to do the sums.
Interestingly, the distance to the horizon calculation is effectively identical to the calculation of orbital velocity.
Enrichment tasks
Find a formula for the distance to the horizon from a height h on a spherical planet of radius R.
Formulate an experiment to find the minimum height from which people would perceive the horizon to be curved rather than a straight line