Here are the corrections for the mistakes in the provided text:

1. **Sentence 1**: "you will have more time for outdoor activities." (Correct as is, but consider "outdoor" style or singular)
2. **Sentence 3**: "has collected so many books for poor children so far." (Change "so much" to "so many" for countable nouns)
3. **Sentence 4**: "recently, this organization encourages people to recycle rubbish like cardboard and paper." (Consider replacing "as" with "like" for clarity)
4. **Sentence 5**: "You should watch less TV because looking at screens for a long time hurts your eyes." (Change "fewer" to "less" as "TV" is uncountable)
5. **Sentence 6**: "The Japanese eat a lot of fish and healthy food, but they live for a long time." (Change "at" to "a lot of")
6. **Sentence 7**: "We should follow the advice of doctors and health experts in order to keep fit." (Change "from doctor" to "of doctors" for grammatical accuracy)
7. **Sentence 8**: "You may not like action films now, but I think you will like them in the future." (Correct "like" to "will like" for future tense)
8. **Sentence 9**: "We built a greenhouse in the school garden last week." (Change "build" to "built" for past tense)
9. **Sentence 10**: "Our team (did not use) any games so far; it’s so encouraging." (Change "not (use)" to "did not use" for clarity)
10. **Sentence 11**: "I might open my mind yet to when I intend to join the basketball team." (Consider rearranging for clarity)
11. **Sentence 12**: "Did you ever meet anyone famous?" - "Yes, last summer I sat next to Brad Pitt on a plane to LA." (Change "this summer I (sit)" to "last summer I sat")
12. **Sentence 13**: "The Browns live in this town since Mr. Brown retired." (Change "live" to "lives" and "retire" to "retired" for past tense)

Please adjust as necessary!