I recommend participating in the Nut November hackathon, especially if you are a newb in the bitcoin app development space:

Nut November

Nut November - A month-long hackathon for Cashu builders

A month-long hackathon for Cashu builders who just can't hold their nuts.

Very cool pull request in bitcoin core:

GitHub

kernel: Introduce C header API by sedited · Pull Request #30595 · bitcoin/bitcoin

This is a first attempt at introducing a C header for the libbitcoinkernel library that may be used by external applications for interfacing with B...

And it's merged! It "may be used by external applications for interfacing with Bitcoin Core's validation logic" It also links to a bunch of cool applications that already use it