Instrumental also fairly useful and easy to learn the rules for
Yes, I agree that instrumental is good to start with. It's easier because doesn't involve "żywotne" and "nieżywotne". But I also agree that it's not as useful as accusative, genetive or locative - the structure S+to be+Instrumetal can sometimes be replaced with S+to+Nominative. However, instrumental is also used with preposition "z", and phrases like "ryba z frytkami", "herbata z cytryną" or "wódka z sokiem" might be useful in everyday life :)
I am new to this forum and cannot send you an e-mail or paste a link to sendspace, but maybe you can contact me. I found some exercises, there are not many of them but I hope you will find them useful. There is no key (I made it for class purposes), but you can always ask if you have any problems. I'll let you know if I find more. Please send me an e-mail and I will send you the document.