How to feed the birds not the squirrels (not to expensively either)

I've been thinking of feeding garden birds for a couple of months now, and have bought feeders, seeds, feeding stations etc. but we have a serious problem with squirrels, even when there aren't feeders there are regularly 2-4 squirrels in the garden, it don't think it helps that we have an oak tree either. I was wondering if anyone had a budget friendly way of protecting the seed from the squirrels as my older brother used to have a feeder out and attracted many birds, including a great spotted woodpecker.

We some trees on one end of the garden (a small oak, an old plum, and a good sized fir tree) plus a bush which runs all the way along one side of the garden. However there is also a trampoline near the trees, and the garden is often in use in the summer, and is open so would that scare the birds away?

Thanks for the help in advance and hope that someone will be able to give me some ideas

Regards

Benji