Keeping a small project small
A useful project can have a modest ending.
Choose one outcome
Describe what you want to finish in a single sentence. This gives you something concrete to check when a new idea appears.
Keep a separate list
Write down extra ideas without adding them to the current work. A separate list lets you keep useful possibilities while finishing the task already in front of you.
Save a working version
When the outcome is usable, keep a copy and a short note explaining how to return to it. Later improvements can begin from a version that already works.