"His lessons are not only fun, but are also very fulfilling."
- "His lessons are not only fun, but are also very fulfilling."
"his lessons" 「彼のレッスン」
"are not only ~ but also" 「〜だけでなく」
"are very fulfilling" 「密度が濃いも」"interesting" 「面白い」や "engaging" も使えます。