"challenge" ideas I was thinking about...more like jokes, I think.
1) The F10 Challenge (MAME Only)
Load up US Set 1, press F10, and see how high you can get. I've yet to grab the bottom hammer.
This one has a couple of problems. First, on a fast machine you have no chance at all so it isn't interesting. Second, F10 has MAME run without throttle, so with a lower-end PC it won't run as fast as on a high-end PC, so not everybody would be playing at the same speed.
In recent versions of MAME though there is a "-speed" option that lets you specify a specific speed to run at. For example if in the command line you use "mame dkong -speed 2" it will have everything run twice as fast. This would actually be a good challenge I think. Note that the -speed option isn't supported in MAME .106 as it was added later, so you can't do this with WolfMAME .106. I'm not sure exactly when it was added to MAME, but it definitely isn't in .106 and it is in .126 and later. Also, if you use the GUI front-end to MAME it may not be as easy to run with the -speed option.
Regardless, I'll propose the actual challenge of playing Donkey Kong in MAME using the -speed 2 option.
-George