Mar y tierra is a traditional playground game. It is very simple and it can be played inside if you have enough space and do not mind the jumping. Traditionally, kids jump between two spaces designated mar and tierra. Since it is almost Easter, you can make the kids conejitos and designate the two spaces bosque or campo and jardín.
To play, kids line up beside a line on the ground, so that they can jump sideways over the line. They can also face the line and jump forward and back. One side of the line is mar and the other side is tierra. A person who has been chosen to call first, calls out mar and tierra as the players jump across the line (or in place) to be on the correct side. The child who makes a mistake and jumps in the wrong place is out, or not, depending on how you want to play.
Here is a video of kids playing Mar y tierra. They play without a line and it works fine too.
Here is video of a group of scouts playing. They are jumping on and off a (not so) low wall. It makes the game considerably more aerobic!
You can see an animated explanation of the game here: Spanish game – Mar y tierra
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