I'm asuming you just mean Ocarina of Time in general?
There are quite a few ways to get easy rupees, I suppose my definition of obtaining them 'easily' may differ from other people's. Obviously you can run and slash tall grass, throw rocks, jump across the stepping stone type things in Kokiri forest. You can go grave digging at Kakariko Village, there's a lot of places, really. Simple things like smashing rocks or falling objects. Or playing various games like the diving one at Zora's Domain. They're all very simple answers. But there are also places with loads of rupees inside barrells or boxes or crates etc.