What can I do?

You can run any Java code using MCJava (although some packages like java.io and the System class are restricted for security reasons — if people want to access those, I can add an option to the config)

For a list of everything you can use, and all class names, just google "java.lang JavaDocs" and "java.util JavaDocs" for java, and for Bukkit/Spigot/Paper, look up "Spigot javadocs".

Latest Minecraft JavaDocs
Minecraft 1.16.*
Minecraft 1.15.*
Minecraft 1.14.*
Minecraft 1.13.*
Minecraft 1.12.*
Minecraft 1.11.*
Minecraft 1.10.*
Minecraft 1.9.*
Minecraft 1.8.*