While I'm waiting for the case I really want, I'm using an i-Blason case from Amazon. $15 for Otterbox-like protection, so not too bad.
What I really want is the same setup I had on my S3. I was using a Hyperion 4200mah battery and the Active Extended case from Seidio. They primarily make that case for use with their 4200mah battery, but their battery is more expensive than the Hyperion. I just picked up a 5200mah Hyperion battery yesterday on Amazon for $22 shipped. I called Seidio to ask about the Active Extended case for the S4 and they said they are only taking requests for it at this time... I guess to gauge demand. Hopefully, it makes it to production, but even if it eventually does, sounds like it's gonna be awhile before it's available.
If you're not into using an extended battery, their Active case is still worth consideration IMO.