Q: What can I do for my mother that passed away so she can be happy?

A: Give sadaqah, perform nafl salaah, if you already did your hajj then do one for her, you can do an umrah for her, recite quran, give good advice whilst sending the reward to her, read tasbeeh and send the reward to her, etc.

As can be noted, any nafl action worthy of reward can be sent to her.

Allah Certainly Knows Best.