Great tips already.
Be prepared for a lot of walking as the animals will probably be about later if sleeping etc but there is a distance to go! If doing the elephants first check on the tiger and marabou storks. Usually the tiger (weird to write just the one now!) is more active at the start or near the end of the day.
Theres an activity / talk board as you enter the zoo and it’s worth snapping a pic so you can go along to things if you want to. Some of the animals will get up for the talks (the otters will if they’ve been in bed earlier).
Definitely second going to the bird shows.
if a first visit I would go on the train - it’s a good orientation to things you can go and find after and it has some nice views.
If over near the hippos you can do a bit of a repeat around flamingoes cheetah lion meerkats aardvark zebra etc if anything is asleep first go and there are some benches near the lions and a series of shaded picnic huts circle around the top of the park there which you might find useful if it’s hot.
Whipsnade signage can be a challenge - don’t be worried about asking where stuff is most people will be happy to help out. I am asked for more directions at Whipsnade than any other zoo I visit and it’s never an issue. I see people point the way all the time.
On a Saturday all the refreshment places should be open and there should be volunteers around too.
If you are taking the car in then passage through Asia has a car park (the Whipsnade mount) near the end which is worth stopping in - you can walk along the inside near the fences and if the horses and foals or the camels are up that end you can get some good on foot views. Sometimes the hoofstock are also near the gates at the top there too. Otherwise if on foot you can walk right to the cattle grids on both sides of the passage through Asia which offer some good angles if anything is near the entrance / exit.
This is really good advice, I think often the challenge for someone new is that they often stick to the road. Definitely venture down paths etc as you will find lots of really exciting animals and great enclosures.