low-key

(adjective) not elaborate or special, ordinary

Example Sentences:

A: How was your weekend?
B: It was pretty low-key. I just stayed at home and watched movies because it was so cold.

I enjoyed my Uncle Peter’s wedding because it was a very low-key event. It was small, the ceremony was short, and it was really nice.

From the Blogs:

Low-key wine and beer bar opens in SoHo

Media:

Similar idioms:

1. start the ball rolling