What are ways to monetize an app with a free plan?

I have an app in which I would like to offer a free plan for users along with a paid upgrade if they want more features. My app is built using Google Cloud and each user cost me roughly $0.30 per month per user. So, per thousand users on the fee plan would cost me $300 a month in server fees.

What are some ways I could subsidize or monetize the free plan for users?

One obvious way is by ads, but I really don't want to ruin my user's experience with ugly ads.