Biking to the Dutch countryside... how to plan?

Just knowing the Dutch countryside, I would say go with the route recommendations from a bike rental company. The tourist image of the Netherlands might be tulips, dikes and windmill, but more typical are factories, modern public housing estates and industrial farming.

Years ago the TI had a really nice bike route, north of the city on the other side of the river. It was the highpoint of my stay in Amsterdam.

North of Amsterdam, about 20 km, is the town of Katwoude--on the outskirts of Volendam. Right off of the main road is the Van-der-Valk Hotel. We rented bikes there and road them into the town of Volendam, then outside of Volendam along the top of the dike to the town of Edam. Yes, on this route we rode past windmills and green pastures with real cows. I have been in this area many times, but enjoyed it a little differently by riding a bike. Here's the website for the hotel: www.van-der-valk-hotel-volendam.com/hotels/umgebungsbilder/ You might be able to take a bus up to this area from Amsterdam. The immediate area around this hotel is a bit touristy in that there is a store where you can see a demo on making wooden shoes and gouda cheese. We bought some gouda cheese there and brought it home with us. Have fun in one of my favorite places in the Netherlands.

North of Amsterdam (you can start at the railway station!) is the superb nature reserve of 'Twiske'. You can cycle very easily through it with this map:
http://www.recreatief-fietsen.nl/routes/amsterdam.pdf
It's only 41,5 km.
More ideas (also in Dutch, but the maps are clear enough):
http://www.nationalelandschappen.nl/cont_files/uitvoerprogramma/9/binnenwerk.pdf
Renting bicycles is extremely easy. A list of bike rental addresses:
http://www.iamsterdam.com/en/visiting/touristinformation/gettingaround/rental/bikehire

Another place to rent a bike is at www.frederic.nl - a couple years ago the bikes were 10/day.
The book Backroads of Holland by Helen Colijn has info about a range of trips. In the countryside, road signs for bikes are helpful. 10 years ago I had a lovely time biking all over the country.

A few years ago I went for a six day ride starting in Houten, south of Amsterdam. I worked through a bike touring company called Tulip Cycling. Check their web site....they will rent you a great bike, plan your route, book rooms and run a transport van to move your luggage from one hotel to another. I had a great experience with them and look forward to the day when I can do it again. Biking over there is great. I am not sure if you just want a day trip, or an overnight or two experience.. their web site will help you . Have a great ride!

You don't need to pay for a tour. Maps of bike routes are available, as others here have posted, from the TI and any bike rental shop. I rented from City Bike in the Jordaan.

There is a pedestrian/bike ferry across the river near the train station. Then you follow the signs to Waterland. Beautiful farmland and easy to bike.

I always start and finish in Houten ;)

You could take a tour, but I think it would be just as nice to ride out on your own. The tourist information offices as well as the bike rental agencies all have maps with interesting cycling routes.

There are also maps that include so called cycle junctions ('fietsrouteknooppunten'): you can create your own tour by cycling from junction to junction. Unfortunately for those who don't speak Dutch, most online information is in Dutch only. There should be some information in English on www.iamsterdam.com/en/ and us.holland.com.