You can hack on whatever project you like, but there is also a friendly competition called The Project Euler Sprint (rules below).
Beginners and practitioners alike are welcome to attend! Feel free to pair up or form teams. Food and drink will be provided.
Project Euler Sprint Rules
The Project Euler Sprint is a competition in which teams complete computational math problems from Project Euler (problems can be found here: http://projecteuler.net/problems). Starting with problem #1, each successive problem is harder than the last. Consequently, each problem has a point value equal to its problem number. Points are tallied up at the end of the event and the team with the most points wins.
1) Problems come from Project Euler.
2) Solutions are worth their problem number in points.
3) Solutions must be coded at the event.
Mathematics libraries with methods for prime, factorial, fibonacci numbers, or other methods that make solving a particular problem trivial are not allowed. Those methods must be implemented by the team during the hack night.
4) Solutions may be in any language.
5) Problems may be done in any order.
6) You may practice ahead of time.
7) Each solution must be checked in* with an event organizer to count toward a team's point total.
8) Teams may consist of up to four people.
9) Participants are given 30 minutes at the beginning to meet, eat if any food has been provided, and form teams. They are then given 90 minutes to solve as many problems as they can.
* Check ins involve showing the code, running it to produce a solution, and showing that the solution is correct in Project Euler's validator. This means there needs to be at least one person per team who has a ProjectEuler.net account.
The Project Euler Sprint by Brian Kung is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Based on a work at https://gist.github.com/briankung/8724819.