To get up to something means becoming involved in something, usually mischief.
Example sentences:
- What shenanigans have you girls been getting up to the past week?
- Where is Jack? I have a feeling that he is not going to get up to anything good.
- Don't lie to me. You and your friend got up to something last night, didn't you.
Also remember the phrasal verb put somebody up to something that we discussed earlier. That means persuading somebody to do something bad or stupid.