Build in flexibility. Who knows what tomorrow may hold? There are ways to build a room so that it can be broken into two rooms if needed. These aren't walls that would be changed weekly, but something that can be moved with less effort than sledgehammers and drywall.

I agree about bathrooms. Our elementary school adapted so that there is one unisex bathroom adjacent to every classroom. Teachers feel that it has given them dramatically more time and peace of mind.

Zone heating. You don't want to heat an entire building if you are only using the youth room or nursery.

Doors with glass and/or glass windows to the hallway (which are not curtained over). Sexual abuse prevention people all will tell you that abuse is much less likely if any passerby can see what is going on in every corner.

Some sort of livingroom-ish corner in a reception area for parents to sit down and chat with each other.

Youth room with pool table or ping pong table, or something that will draw them in.

Don't forget the adults. They need good interesting usable spaces for Bible Study, too, if you want your adult program to grow.

Lucky, lucky you say those of us in historical downtown properties.

Peace,
Lisa