Terminals like CMD have support of navigating through commands with ↑↓. While directly implementing the arrows is not good in IDLE (the use for jumping to the prev. line in GUI is needed), there should be a good way.
Some ways:
1. Alt+↑↓. The current behavior is exactly like ↑↓.
2. A "Previous input" option in the right-click menu.
3. A "Navigate" option in the right-click menu. A GUI like this will pop up:
——————————————————————————
┃        Navigate        ┃
┃  × The [4]th command   ┃
┃  O [1] command before  ┃
┃        _______         ┃
┃        ┃Paste┃         ┃
┃        ———————         ┃
——————————————————————————

It would be better if 2&3 are together.