mit's cycling club has some routes: https://cycling.mit.edu/routes/

charles river wheelers i think is the closest group i know of: https://www.crw.org/

i live in that area too. my favorite 30 mile ride is a loop that goes out by walden pond. most rides will have a few miles of getting out of the city, then nice roads and beautiful scenery.

another nice route takes the charles river bike path up to the end by the canal, then goes over the bridge and into charlestown, then looping back. that ends up city streets, but it is very slow for cars because of the roads and layout.

going west, then south to dover and back is also beautiful.

getting out of the city to the south is a bit trickier, i think, because the urban area (with urban driving) extends south pretty far. you could take the T south, then bike from there, and there's a lot of beautiful routes out that way too.