A white noise machine -- our bedroom overlooks a busy street, so there's constantly random noises in the middle of the night. Big improvement in sleep quality!
We got a Dohm Classic, but you can just get an app for your phone if you like.
I've been using an app (Noisli, before that it was White Noise Generator) on my iPad every night for the past 6 years.
Works great and you can customize the white noise. At first I was concerned that running it continuously for 8-9 hours every night would wear out the speakers on the iPad, but that was an unfounded worry.
Living in CA, I bought a big air purifier last year. I found not only did it help my breathe better, if I left it on medium or high, it's a good white noise machine too.
We got a Dohm Classic, but you can just get an app for your phone if you like.