General:
* Engaging
* Interesting
* Exciting
* Stimulating
* Invigorating
* Thrilling
* Entertaining
* Amusing
* Fascinating
* Captivating
More specific:
* Lively (for something energetic)
* Dynamic (for something fast-paced)
* Varied (for something diverse)
* Challenging (for something intellectually stimulating)
* Rewarding (for something fulfilling)
The best antonym will depend on the specific context in which you are using the word "tedious."