I would also recommend St. James Children's School on Seven Locks.

We ended up choosing a school near my workplace, not in Bethesda. If we had stayed nearby, I would have chosen one of the top 3 in that list. I think Harbor School is closest to the approach of the school we chose (and also has the benefit of smaller classes). I was so impressed by their director. Geneva Day School has one class that's all day and mixed ages that I was interested in too. Oneness Family School is neat because it mixes aspects of Montessori and Waldorf. If you go to the Open Houses, you can see where you're comfortable and if the parent community seems like a good fit. Observing the classes I found to be extremely helpful too.
Another place I liked was Grace Episcopal in Silver Spring. Very nurturing and warm for the youngsters, and very small classes.

Chevy Chase Reggio at Connecticut and Bradley is a full day preschool and also offers a bi-lingual (English/Spanish) program. CCR has an opening from a family relocating for work and will have an open house for the spring on Saturday, November 5th from 10 am to noon. For more information, email info@ChevyChaseReggio.com

Apple Montessori off River Road is a very nice school.

St James on Seven Locks is wonderful

You have missed Family Academy of Bethesda which has a great Montessori Program.!My daughter is in the Primary Montessori, standards are very high compared to other Montessori schools in the neighborhood.
Do check this out.

OP here - Thanks everyone for the advice.

14:24 -- Can I ask where you landed? Thanks for that great list!

Clara Barton Center for Children has full day/ full week options

There are so many. depending what you are looking for:

Geneva Day School (technically Potomac, but nearby)
Harbor School
Oneness Family School
Walden Montessori
Manor Montessori
Children's House (montessori)
Washington Waldorf
Wonders
Children in the Shoe
Clara Barton (not sure of the hours)
Lone Oak Montessori
Woods Academy (montessori)

Also check out the affordable programs here:
http://www.mccaedu.org/child_care_programs.html

Personally, I did not like any of the Bethesda montessori programs I looked at and have ended up at a play-based preschool. (I also didn't like Bethesda Country Day). Good luck! It's sort of fun looking around, but I'm glad we're done with it and are thrilled with where we've landed! If you look all around, you're sure to find a good fit for your child and you.

Feynman school

Rock Spring Children's Center

My DD is in her second day at BCD and we absolutely love it. I only know of one family in our class that wasn't happy but other than that the parents I know seem very happy. I think years ago there was a problem there but I think that has been fixed.

YMCA Ayrlawn center

Kenwood Park Childrens Center
Westmoreland Childrens Center

My DS is currently in a morning preschool program. Next year, it will be a five day program, mornings only and our nanny may not be able to continue working for us with the reduction in hours.

I'm thinking of moving him to a full day preschool (or at least one that goes to 3pm). I've looked at Bethesda Montessori and it didn't feel quite right (large class size, etc.) and I've heard mixed things about Bethesda Country Day. Also can't spend a fortune. Any great, affordable full day preschool programs I'm missing? Thanks!