Almost anything actually - If you have a compatible phone. There is a edition of kali linux called nethunter, which can be installed on a phone, and supercharges its hacking ability. It also comes with many common built in tools that kali comes with so it's very useful. https://www.kali.org/kali-linux-nethunter/. Alternatively, find a phone with OTG and use a pi zero with a kali image.

After getting kali, there are many tutorials online you can use. One channel i frequently watch is:

https://www.youtube.com/channel/UCgTNupxATBfWmfehv21ym-g - Null Byte

Again if its Android, install Termux (a command line linux) and then CLANG (a C compiler) and then try your hand at coding in C on your phone.

Assuming you have an Android phone, take one of those free Android courses and program for your phone. Enhances your job prospects too.