For tasks that are generally unpleasant:
* Pleasure
* Joy
* Delights
* Hobby
* Pastime
* Entertainment
* Fun
* Luxury
For tasks that are routine and required:
* Luxury
* Freedom
* Relaxation
* Leisure
* Recreation
* Vacation
For tasks that are specifically burdensome:
* Reward
* Benefit
* Gift
* Treat
* Privilege
The best antonym depends on the specific meaning you're trying to express.