What gear should I use when riding my bike in the park?

Question

I have a mountain bike; is this bike appropriate for leisurely park riding?

Answer 1

Whichever gear feels best to you.  No one else can determine that for you.

Answer 2

Most humans develop maximum horsepower between 90-105 rpm.   The purpose of gears is to allow you to pedal in that power band.  

Of course that isn't terribly relevant to a leisurely ride, but the point is that most recreational riders make things difficult for themselves by trying to push too big a gear at too low an RPM.

A mountain bike is just fine for general purpose use.