Nice idea. Here's what comes up to my mind:
Sun
- Clouds lit from below at sunset
- Shadows climb buildings during sunset.
- Mountains cast shadows from below under the clouds at sunset.
- Sun getting red at sunset, that is predicted in RE by inclination of Sun rays w.r.t. atmosphere scattering
- Reflection of the Sun on the sea has an edge on the horizon.
- In some pictures it is possible to see that sun rays are parallel. Example, shadows of clouds are as big as the clouds.
- The horizon is a crisp clear line, as if ocean is frontally curving down. On flat earth there should be a brownish band of confused far landscape.
- Equinox has the Sun coming straight in the sky, no way it's hovering around us.
Geometry
- Spherical geometry of distance (maybe it's already there, but afaik this is the most uncontroversial one, that FEs dodge simply by refusing to take... distances.)
- Horizon dipping with altitude, that can be checked simply using a bottle of water on a plane.
- Pictures of distant mountain tops, where tops of background mountains are much lower than what perspective would predict.
- Travelling East on the Equator going in a perfect straight line
- Visibility of Polaris, its altitude being your latitude
- Radio transmissions cannot go very far
Miscellanea
- Foucalt's Pendulum/Laser gyroscope expertiment
- Pontchartrain pictures
- Gravity varying with altitude