It's a question I was asked as a child, and when I was small I always had the same answer "visit the London zoo".
As a family we were poor as church mice, but somehow my parents always found the money to manage the trip, plus on a couple of occasions in included travelling on a boat, right into the zoo itself. Having just enjoyed a little remembering back to childhood, it was natural to spend 30sec in google, and lo & behold, it's still possible.
We never did this for my son, and at this point in time I sorely regret it (we were dirt-poor too at that stage) although he might never have actually been interested*, but no matter. Perhaps I'll get to take the grandkids sometime.
*Retrospective regret is a useless fantasy. I frequently beat myself up with it, when the reality is that a) we did the best we could at the time and b) Cadbury world was of greater interest then. I hope he has good memories of those trips, like I do of the trips my mother took me on.