We have loved this zoo for years! Admission to the Como Zoo in St Paul, MN is free (they ask for donations - but they are not required). For being a free zoo - it is great! They have giraffes, zebras, tigers, lions, fish, seals, flamingos, polar bears and more!
They also have a small park called "Como Town" with some rides mostly for kids - such as a frog hopper, mini roller coaster, etc.
They also have a conservatory that is very well maintained. They have plants from all over the world. It is nice to visit on a cold day and gets pretty hot during the summer.
The downside - being a free zoo - it is VERY busy on the weekends! Parking is hard to come by - so come prepared to park on a city street and do some walking.