If you're going to Södertälje and have some time over, take the road via Botkyrka - Vällinge - Viksäter instead. The first part is real narrow and winding. It could be real tight if you meet a car why you should be prepared for it.
If you instead go via Drottningholm, and the castle were the Swedish king lives, you can choose between both Färingsö, Ekerö and Adelsö. On Färingsö there's a lot of small roads to choose upon. On Ekerö you'll find the first part quite straight on. The really nice one's you'll find on Adelsö.
Botkyrka - Vällinge - Viksäter - Södertälje | Drottningholm - Ekerö... |