The Moon: Whenever there is a lunar eclipse there is a shadow that can be seen on the Moon. That shadow is the Earth and the shadow is round: telling us the Earth is round.
Ships and the horizon: If you look at the horizon and look at a boat coming towards the shore it does not just appear out of the horizon, which it what would happen if the Earth was flat but it appears to rise out of the sea, a phenomenon that shows the Earth is round.
Constellations: The constellations of stars you see in the sky vary depending where on Earth you are. The explanation for this is the Earth is round and depending on where you are constellations are above, near or below (cannot be seen) the horizon. On a flat Earth you would be able to see all constellations.
Shadows and sticks: Stick a stick in the ground and it will produce a shadow. If you stick two different sticks in the ground in different locations the sticks will produce the same shadow. In practice, the two sticks produce different shadows and this is because the Earth is round.
Height: The higher you go up the further away the horizon becomes. If you stood on the ground you would see a horizon. If you were on the twentieth floor of a building the horizon you saw would be much further away. This is the result of the Earth’s curvature. On a flat Earth a higher vantage point would not enable you to see further.
Flying: If you have been on a long-haul flight you may have noticed that planes can fly for a long time without falling on the edge of the Earth. Indeed, planes can circle the Earth without stopping. If you look out of a planes windows you can see the curvature of the Earth at the horizon.
Other Planets: All other planets we have ever observed are spherical. We also know how planets form. If all other planets are spherical why should the Earth be any different?
Time Zones: At any one time it is light in certain parts of the world and dark in others. This happens because the Earth is round. If it were flat we would have sunshine all the time. Even if you wanted to argue the Sun is like a spotlight only shining on one part of the Earth we would be able to see it even in not directly shining above us.
Gravity: Gravity pulls you towards it. On a sphere the centre of gravity is in the centre of the sphere and anything on the sphere is pulled downwards towards it, exactly how gravity acts on Earth. On a flat plane the centre of gravity would be in the middle and anything towards the edges would be pulled sideways towards the centre and not down. This is not how gravity acts on Earth.
Images form Space: There are numerous images taken by astronauts and satellites from space of Earth and these show the Earth is round.