I have hemmed and hawed over whether to make this a general post on free zoos (or pet shops/garden centres as they're more commonly known) or to talk about the one we specifically go to. There are a few in our area and they all have different merits but I've decided to plump for talking specifically about our favourite, which is Burston Garden Centre in St Albans. I'm sure there are some general principles that I hope will be of use to you.
Logistics: It's easy to get to on the A405, well signposted from both directions.
Activities: In the garden centre itself there is a very large and slightly terrifying animatronic owl with small chairs in front of it. When your kid pushes a button and takes a seat the owl will tell a story. The story never changes; my kid doesn't seem to mind this, she will listen to the story up to five times in a row. I don't know what it's about... something about bees I think - I'm too busy looking at all those lovely gewgaws you only find in garden centres and boutique home shops in zone 2.
The real attraction though is the aquatics and reptile centre which has an astounding variety of creatures. They have skinks and snakes, turtles, tortoises, giant snails and bucket loads of fish. They even have axolotl god dammit. The staff there are willing to humour me when I pretend I might buy something. What I love most through is that you can pay a quid and get some fish food to chuck to the koi carp outside which legitimises the whole thing and assuages my guilt nicely.
Playground: There isn't one - if you want a free zoo with a playground try the St Alban's Maidenhead Aquatics, which is just up the road on the A405 and has a soft play in the cafe area.
Food: It's a bit pricey but the staff are lovely and the kid's lunch boxes offer a good selection to choose from and each one comes with a pack of cress seeds so you can make an egg head at home, which is a nice touch.
Grounds: The aquatics centre and the garden centre are separated by a car park so, if your child is a runner, you'll want to keep a tight hold on them.
Accessibility: It's a garden centre - wheelchair users are their bread and butter. I think kids with ASD or ADHD might find it a bit challenging as there are lots of things that look like you should fiddle with them but you'll probably be told off if you do. Also direct interaction with the wildlife is frowned upon.
Overall: As I said, it's our favourite of all the free zoos we have been to.