Besiege has an official challenge that is hosted monthly by a modder and frequently featured in the official twitter, here is the list of official challenges. You can still do the older challenge if you're bored.
All Challenge Edit
November 2015: Rube Gold Berg Challenge
- Create an awesome rube Goldberg machine
October 2015: Animal Challenge
- Make a majestic animal
- Optional: Bonus point if it's a Cat
September 2015: Spacecraft Challenge
- Create a neat space ship for zero-G with equal to or less than 75 blocks
- Optional: Spaceship Uses Steam/Water Cannon as Propulsion.
August 2015: Crane Challenge
- Create a functioning crane that can rotate and raise and lower something along the Z axis. Movement along the main arm of the crane is optional. (No Block Limit)
- Optional: Crane is able to finish Zone 6, 12, 16.
July 2015: Drone Challenge
- Make a drone with maximum 100 blocks!
- Optional: Drone has a 4 main propeller in symetrical position.
June 2015: Maze Challenge
- Build a maze. Bonus points for building it large enough that a miniature 4 wheel vehicle can navigate. (No Block Limit)
- Optional: Maze Don't have to include 4 wheels vehicle, only require a creative way to make it move.
May 2015: Design Challenge
- 50 block limit, build something cool
- Optional: Machine is Aesthetically Functional
April 2015: Automatic Challenge
- Complete all levels with fully automatic vehicles. (No Button Pressing, Within Boundary, No Block limit)
- Optional: Don't use Automation Mods
Alright starting it up again 2017
May 2018 Speeder challenge
Create a hovering or flying speeder zero gravity no mods
less then 100 blocks