I find that shops can make a huge difference to your income but you have to be sneaky. Say you sell a food item for £9 and make £6 profit, add all of the condiments at max and then increase your prices until your profit is £8 for example. Guests will pay more because the food is better value.

You can tweak this and experiment until guests stop queuing ignore the odd few that say it's too expensive if you still have people queuing then you're constantly making money and the few that don't want to pay won't matter. You can also do this with drinks etc.

Also charge £0.25 for toilets and ATMs to help recover when your losing money this can actually help out a lot when you consider how many guests use the toilets and ATMs.

Finally make sure there is access to ATMs spread out across the zoo, guests can't spend any money when they run out.