I'm a newish DM playing a fantasy RPG similar do D&D but a lot simpler, either way, it won't matter that much.

I'm not very good at improvising yet and if i do it too much it may become a mess thas is why i prepare a session usually thinking of every possibility of actions the players may do and i try to make a route out of it.

In the campaign there is a "imployer" NPC that tells them to find mystical artifacts for the greater good I already thought of what they should do in the next sessions, but its mostly "NPC says to do that, NPC says to go to a dungeon and lick something from there".

The thing is, i feel like the adventure is too linear and usually they just do whatever the NPC asks them to do, i may not have given enough space or opportunities of choices, but i want to encourage the players to play outside the box and influence the world with their own actions, what should i do?