My dogs love(d) to chase water from the hose. They kept really busy doing that, especially when it's warm out. Plus icecubes are great chew toys and good for doggy airconditioning, too.
My ACD will not drop the dam ball either, UNLESS I have another squeaky ball, in which case we trade balls every time. I don't know if you can use squeaky balls with Rotts. tho. There is a toy that stands up to my ACD (hey they can bring down cows, they're tough) and they are the airKONG yellow-fuzzed squeaky balls. If the ACD can't kill it, the Rott won't either. Try two of the small Squeak footballs. You can squeak one and throw it and then when she brings it back, squeak the other until she drops the one in her mouth and throw it and so on. Most balls don't last a day with him, so trust me, these are TOUGH enough!!!
If we had a timer on who destroys balls faster, who would win? might be a toss-up!