My geass would be the ability to influence machines (like there programs and there actions)
side effect/risk: gets sleepy or tired more easily/ headache / or just plainly physical exhaustion
warning:

Hacking into a computer is possible with this geass but requires you to be concentrated and you will get really tired when you are done hacking (tired meaning being sleepy).
OR
The ability to make anime characters come to life and make them do whatever you order them to.
side effect/ risk: your character can only stay for 1 hour and you can summon only up to 3 characters and up to 3 characters can stay at once and for every 3 characters you bring to life you need to wait 1 hour for your ability to recharge.