My 4 year old's favourite rides were the log flume, Viking river rapids, Apprentice rollercoaster (smaller one in the Knights area) and fire academy. Don't bother with the Laser ride in the Pharaoh section - huge, slow moving queue and he only worked out how to aim at the end of the ride!
Use the lockers rather than going back to your car for picnics etc as it's a long walk so will waste lots of time.
A couple of the drink stalls are marked as Hydration Stations on the map and the staff there will fill up your water bottles. No need to buy the drinks cups if you just want water.
Start at the back as others have said and stay till closing time if you can.