You can actually do this pretty easily with events.
Simply put up a 'show choices' and rename each choice to the quest. After that within each choice allow the player a description of the quest and then give them a 'yes or no' to take the quest. If yes, then turn on the switch for that quest. Then the switch should allow the quests to start in a different part of the game.
I would personally also put different pages on your sign, so if one of the quest switches is on they cannot take another. Unless you want them to have the option to take them all at once, which can get confusing for the player.