I want to first say that I second everything FAST6191 said. He covered everything I would have said and more, in a much more concise manner than I would have managed (I tend to ramble). A few things I wanted to add to the discussion:
Yeah, the DS homebrew scene was friggin awesome and I miss those days of coming home each day after highchool (later college) to check out GBAdev/GBATemp/PH/DCemu/Emuboards/others and test all the latest homebrew and stuff. It's hard to imagine a world without Android or iOS nowadays, but before those came out, the DS (and PSP
I suppose >_>) were the kings of handheld gaming. Turning my DS into basically a smartphone (I even made a successful VOIP call via SVSIP!) before smartphones even existed; those were fun times.
As FAST mentioned, for video your main one is going to be Moonshell and converting the videos to DPG format. For audio, the DS is actually powerful enough to handle straight mp3 decoding among many other formats -- lots of chiptune formats for example. Nice thing about Moonshell is that by default it does turn off the screen when the lid is closed, and you can use the L+R buttons to play/pause/next/previous. I used this feature a ton when I used my DS as my car's mp3 player, as I could just blindly grab the console without taking my eyes off the road and click to the next song. And by requiring both shoulder buttons to be pressed rather than just one, it also worked in my pocket as an mp3 player without accidentally skipping tracks.
One audio player not mentioned that I enjoy is called "LMP-ng" which stands for Lick's Media Player... Next Generation or something I dunno. It's fun in that it's an iPod Classic clone, using the click-wheel interface with your DS stylus and everything. Inefficient yes, but it's a lot of nostalgic fun if you ever owned an iPod Classic; plus, it can shuffle the entire music library on your whole microSD card which is handy sometimes (Moonshell can only shuffle within the current folder). You'll need to make sure you download the skin files and put them in the right spot on your SD card or else you won't be able to see anything.
Other homebrew I'd suggest: Do you like sandboxed playgrounds? Check out "World of Sand DS" or "Pocket Physics". Like the Portal games? StillAliveDS is a great 2D adaptation, plays like the Flash version. Love to draw? "Colors! DS" actually utilizes pressure sensitivity for drawing (official DS games were not allowed to do this because of differences in touchscreen hardware versions over the years, which the homebrew gets around by having a built-in pressure calibration menu), and it actually went on to become a
commercial success! If you like rhythm games there's VideoGamesHero and AmplituDS. There's WabbitDS for emulating the TI-83+ calculator among possibly others, DSx86 for running DOS games, even stuff for Neo Geo and Wonderswan (may not be compatible with your R4). There's DSReader for e-book reading, and this one is special in that it can import standard Windows *.ttf font files, alongside some other good features -- I read the entirety of the 7th Harry Potter book on my DS via DSReader, white text on black background, Arial font size 10, medium subpixel anti-aliasing, much more pleasant on the eyes than you'd expect! And of course there's DSLinux, a full-fledged operating system port to the DS, but without any slot-2 RAM that's not going to be very useful for you on your R4 so I won't go into further detail on that.
For image viewing, Moonshell will be able to handle your standard everyday images, but perhaps slowly depending on how big the image is -- 66 MHz is not a lot of processing power to work with. Also, if the decoded image can't fit into the DS's meager 4 MB of RAM then there's nothing Moonshell can do about it. Instead, check out the "Comic Book DS" program, which sadly does require you to pre-convert your images into a special format on your computer first, but once done it's by far the best way to view large image collections on the DS. Good for comic books, as the name suggests.
Getting back on track, I'd agree with FAST that 16-bit emulation is going to be a bit hit-or-miss depending on your expectations. I myself did tons and tons of testing with SNEmulDS so I'm a bit partial to it. As far as I know, it still remains the only SNES emulator for the DS with support for the SNES Mouse being used via the DS's touchscreen. That said, lots of games are quite playable as long as you're okay with changing some graphics settings manually throughout gameplay, and in fact SOME games do work perfect...ish... As for jEnesisDS, I just wasn't a Genesis person myself so I could never get too much into it. 8-bit stuff on the DS is great. I played tons of Lameboy for GBC although there are other options out there nowadays. NES DS is awesome, I absolutely love being able to rewind/fastfoward in real-time via the L and R buttons, which is the only way I ever managed to beat the
Super Mario Frustration romhack (which does in fact exist, I can provide the .ips patch file if anyone's interested, you must provide your own ROM).
Like I said, I tend to ramble, my apologies hehe. I had a real passion for this stuff back in the day, and even though it's now literally 5-6 years later, I never quite managed to shake my excitement over it.
EDIT: "adaptation" not "port"
