Well I don’t think you can go wrong with The Grapes of Wrath. It is a story with sadness, but not without hope and perseverance. I will say that I agree that Steinbeck can get too dark – as someone said above. The Pearl – if I could go back and not read that, I would.
Thanks to your question, though, which led me to a quick read about Steinbeck on wikipedia, I discovered that he wrote a version of the King Arthur story – after reading the sample in Amazon I think I must read it!
As for choosing some that I have not read – Travels with Charley: In Search of America sounds interesting – he drives across country with his dog. I would also seek out at book of short stories. That would be a good Steinbeck survey, I think, if you could get through it all.
Please let us know what you choose, and how you like it.