Hi LoLiz. Don't know what kind of rider you are, or what resources you have, but:
The Bike trail that runs along the Charles into Watertown is a pretty nice ride on a Sunday morning, maybe ten miles (?) round trip. A stop in Harvard Square for coffee is a nice little perk, too. At the Watertown end, you can also break away and head west thru Watertown to Newton and beyond.
Weston and surrounding area has some outstanding roads, but the hills call for a degree of fitness. And you need a way to get out there. MANY riders on the road on weekends. Just park in Weston Center and go.
The Bike trail from Alewife is also pretty nice - used to ride it almost every other day. 25 miles round trip, but it can get crowded - especially later in the day on weekends. Which is a chronic problem with rail trails. And look out for kids! BTW, you can actually start this ride at Davis Square and cut through to Alewife. I forget the town where it ends, but there's a great little hot dog stand and a a nice bike store at the far end. Perfect. You can also head west on some pretty nice roads out there.
There are also some nice rides in the Newton area. I used to ride out near Brandeis quite a bit. Would occassionally head out to Natick, Dover, Wellesley, etc.
And don't forget the Cape Cod Rail Trial. 50 miles round trip. Sure, it's a rail trail so you can't ride too fast and hard, but it sure is a nice ride. 'Course, you gotta get down there somehow first...
And there's the Pequod Press book, something along the likes of "short bike rides in eastern massachusetts" that shows some pretty decent routes. For instance, I did the Ipswich ride they discuss, and it was pretty nice.
Lastly, there are club rides most weekends, which give you a chance to ride and explore with the locals.
Sent me an email if you want more specifics on any of this.