The stomping of the various YT frontends was a direct result of the API changeover, yes... a month ago, they all still worked. I hate Youtube a whole lot, not gonna lie.
But anyway, pertaining to the original subject matter, whilst not wanting to detract from the intent, this isnt as much of a challenge as it might seem. Any new-world G3/4/5 machine can be used fairly functionally still if it has the resources to run 10.4 or if you are willing to run OS9 or programs in Classic mode.
Due to the number of die-hard PPC advocates, there are a lot of open-source solutions around still being developed particularly for the G5 world where the abrupt Intel switch and subsequent rapid deprecation of support by Apple and third party vendors hit hardest. Indeed the G5 is still a quick machine 10 years later and I use a 2.0 Dual-Core daily, with the only problem I face (aside from the small, and in my applications irrelevant, issue of security risks) is that I can't use Skype, and that really isnt a problem either as I stopped using it around 7 years ago anyway. As far as video-editing, Youtube playback, browsing etc goes, it's a walk in the park, and many other things are just as much a breeze with a multi-core or multi-CPU G5-based machine. So whilst this would be your best bet for pure functionality, as far as being a challenge goes, it really is a bit of a non-event.
Any faster G4-based machine set up right can still be fairly comfortably used as they will run 10.4 fairly well and therefore it is still fairly easy to find relatively current or still-maintained software in the open-source spaces, You can still do most things, although Youtube will likely be a bit of a dog on the slower end of the quick G4 machines... other than that though it still will be not overly challenging with fairly basic needs in mind, simply a little slower than you are used to. Slower G4-based machines or the quicker G3 machines are a bit more borderline, simply by virtue of 10.4 runs terribly on slower machines particularly those with modest graphics performance. That being said, I only recently retired my Yikes G4 to secondary duties as it still performed well until the web got so fat that I needed TFF or Clasilla to browse it, which required 10.4, which is quite a poor experience even with a 500Mhz Sonnet upgrade and RAM maxed to 1Gb. Add to that, TFF was unstable at the time, and Classic mode is messy in my eyes, as is dual-booting, I simply opted to replace it rather than get Classilla. That being said, if it werent for that personal hang upI would still probably have kept it in service. As for video-editing, well... that's what I used it and my B+W tower for and they did the job well and still would now within reason, given enough hard drive and RAM to do so.
If you wanted a particularly challenging challenge that is still , I would suggest either using a 600Mhz G3 iMac or icebook as they give you a reasonable (by G3 standards) level of performance, and are bootable from OS9 to OS X10.4 meaning you have a fair scope of versatility with which to taylor the machine to meet your desired outcome, whilst still running within some fairly restrictive and largely insurmountable hardware limitations that will test your ability to set the machine up efficiently and also test your patience during the usage period.
That being said, as I said there are lots of options available to keep some very old machines viable so whilst not wanting to play down your intentions, it isnt so much the actual use of the machine that would be the challenge, but more setting it up to acheive the desired outcomes with reasonable efficieny and reliability. For what it is worth my 17 year old Powermac 9600 is still a fast and sturdy machine for daily use with some well-selected upgrades and software and has the ability to do most things I do with the G5 on a daily basis (not Youtube just yet)... even to edit video if I really please. Hell, I used a 7600 for exactly that task many years ago. Nowadays, the use of the machine is really not a challenging experience as under OS9 it is nearly as fast as the G5, and I actually use the 9600 regularly. However setting it up to be a functional secondary daily driven machine would have been the challenging part if I had to do it all from scratch (which luckily I didnt entirely have to), as even the simplest of upgrades that make this machine a weapon (like say finding 512Mb of RAM for a beige Powermac are hard to get.
That being said, if you were to use a PM6360 for say a month for your daily computing without chucking it out the window, I'm sure I would not be the only person here whom would be suitably impressed.
ANyway.... there is some dreary 7am sentiment to chew on anyway.