Formal:
* I am at home resting.
* I am relaxing at home.
Informal:
* I'm chilling at home.
* I'm just hanging out at home.
* I'm taking it easy at home.
If you want to emphasize the relaxing aspect:
* I'm taking a break at home.
* I'm having some downtime at home.
Choose the option that best suits the situation and your level of formality.