General Satisfaction:
1. Satisfied
2. Happy
3. Pleased
4. Gratified
5. Fulfilled
6. Contented
Lack of Complaint:
7. Satisfied (again, as it implies no complaint)
8. Quiet
9. Peaceful
10. At ease
Information or Material:
11. Material
12. Substance
13. Text
14. Matter
Capacity or Volume:
15. Volume
16. Capacity
Other Relevant Words:
17. Peaceful (implies a state of calm and tranquility)
18. Serene (implies a sense of stillness and peace)
19. At peace (implies a feeling of calmness and tranquility)
20. Composed (implies a state of being calm and collected)
Note:
* Some words like "satisfied" appear multiple times with slightly different connotations.
* Finding exactly 15 *completely* different synonyms for "content" is challenging due to the broadness of the term.
* The context in which you're using the word will help you choose the most appropriate synonym.
Do you have a specific context in mind? Knowing that will help me provide more relevant synonyms.