These projects are achievable in a weekend and teach you the most important API patterns:

1. Document Q&A: Upload a PDF, ask questions about it — embeddings + completions (covers 80% of real use cases)
2. Meeting Summarizer: Record audio → Whisper transcribes → GPT-4o summarizes with action items
3. Image Analyzer: Upload any image → GPT-4o vision describes it, answers questions, or extracts data
4. Custom Chatbot: Chatbot with a specific persona and knowledge base, deployed as a web app
5. Content Generator: Input a topic → outputs a structured blog post with title, outline, and full content

Start With #1

Document Q&A teaches embeddings, vector search, and chat completions — the core of 80% of AI applications.

Reference:

OpenAI tutorials

https://en.wikipedia.org/wiki/Special:Search?search=5%20Projects%20to%20Build%20With%20the%20OpenAI%20API%20This%20Weekend