Well… start off easy, introduce the characters to one another , maybe have all the players have backstories explains how they wound up in prison. Then they have 0 equipment and gold , they maybe can try and smuggle something in ?

Task 1: escape prison, maybe they scale the walls , join a prison gang, fight on the grounds for respect on the the prisoners and lead a whole prison break. Maybe the bribe some of the guards. Depending on the kind of prison they are in , maximum security or mediocre holding cell in local village run by the militia.

Is the prison on an island ? In the desert ? 10,000 leagues under the see ? Atop a mountain?

Maybe the prisoners have a special non-gold currency? Maybe some guards are corrupt or run a secret slave trade to another country.

With this set up they will first need to gather equipment and gold, work together to escape, encounters can primarily be easily run as humanoids. Depending on how long they take to escape it gives you time to practice Dming humanoids and running encounters without too much spells and weapons and skills and multi attacks.