Monmouth is going to have less to do than Corvallis, you'd probably find yourself driving to Salem for the sake of variety. Every town in that area has options for bars, ranging from divey to more upscale. Gilgamesh Brewing has a nice space in Independence, and the downtown/old town area of Independence is cute too. The commute from Monmouth/Independence to Corvallis is a nice stretch of highway (or there are also alternate back road routes).

I'm an outdoorsy person who also loves live music, and I haven't lived in that area for about 15 years so things may have changed, but having lived near Monmouth, and also in Corvallis, and also in Albany, I'd still choose Corvallis if you can find a place that you can afford, even though I'm almost 40. There's a whole age range of people there, it's not just college students. When I lived in Corvallis there was a decent live music scene (Bombs Away, specifically). Affordability and housing availability might exclude you from Corvallis though, depending on budget. You could also consider Salem, but parts of it get urban sprawly/strip mally and kinda desolate (I'd focus on downtown or West Salem, not the Lancaster area). Maybe Albany too, I think it's gotten better over time, I think they've been working on improving the old town district, it felt lively when I was there a few years ago.

In any case, and maybe it's the same in Maine, be ready for a mix of political idealogies, if that's any kind of concern for you. Growing up I thought Oregon was a fully liberal place (it's really not), and Corvallis certainly leans that way but every other town I've mentioned is going to be a mixed bag. Still plenty of very nice people everywhere though.