What activities should I do with patella tendinitis?

Gentle stretching. Once you've iced, taken some anti-inflammatories (like ibuprofen, naproxen) and rested a little (avoid running and jumping initially after an injury), then you can work in some gentle quad stretching and strengthening exercises. Eccenentric quad exercises work well. Things to avoid: deep lunges. Lunges don't make kneecaps happy. Good luck!
Options. Treatment options include nsaids (oral or topical), ice, brace use (chopat patellar tendon strap), and exercise modification. Specifically, eccentric or negative quad contractions have been shown to be particularly helpful. You could see a physical therapist to help you with all of this as well as try other modalities such as ultrasound.