Artis is an old zoo, some buildings even date back to the 1700s, but it fully meets modern standards.
It's located just outside the center, about 10 or 15 minutes walking distance from Waterloo square.
Artis is quite large and hosts most of the animals you might expect in a zoo this size. There are many newly build areas and there are also some interesting museums and a large aquarium. The planetarium is updated and renovated in early 2007 and worth a visit alone!
Unfortunately admittance is not cheap. Children are 14 euro's and adults 17,50. You get a reasonable discount after 3.30pm, though.