A zoo membership is something that we utilize year round. When the weather is nice, we enjoy the outdoor exhibits. When it's really hot, or really cold, or raining, or whatever, we venture indoors.
Because we know that we can come back whenever we'd like, there's no pressure to "see everything." There have been times that we stopped by to visit just a few exhibits. I remember at least one trip where we only went to the playground.
Besides free entry any time we want to go, our membership to Nashville Zoo also gives us free or reduced entry for a number of other zoos and aquariums. If you have or are considering buying a membership to any zoo or aquarium, see if they're a member of the AZA--the benefits might surprise you.
Budget suggestion: if grandparents are asking what you or your children would like for an upcoming birthday or holiday, memberships that you can use all year make a great gift!