Birches is in Lincoln, MA. They're small and fairly new, but closer to Cambridge than Sage. I think they're more laid back than the others mentioned, but I could be wrong.
There's a Montessori School in Lexington (
http://www.lexmontessori.org/home) and a Waldorf (
http://www.thewaldorfschool.org/) - they're closer to Cambridge too.
There's some schools out in Framingham area that you might want to consider too - though they might be too much of a hike for you. There's the Macomber Center and the Sudbury Valley School (
http://www.sudval.org/).
Bay State Learning Center (
http://www.baystatelearning.org/) is in Dedham and might be doable. I think they're fairly new.
Odyssey Day School in Wakefield (
http://www.odysseydayschool.org/) is another one you may consider (or not). They have some gifted kids there.
Phoenix School in Salem, MA is a small, open one-room schoolhouse and Montessori-ish/ gifted school, but again it might be too much of commute from the Cambridge area.
Another alternative is to consider joining Voyages out of Acton (co-op that has gifted kids) and the growing un/homeschooling community around Cambridge/Arlington greater area.
