You will need to do one or the other (or you can do both), in order to play downloaded games on the wii. If you don't want to open up your console, then you should do a softmod which only modifies the software in the system. Although, a softmod is easy for Nintendo to patch, and they do patch them from time to time which stops them from working.
To stop that from happening you must not update your wii, obviously new games may require updates to install in order to play them. But you can find programs that will update the wii, while not rendering the softmods inorperable.
On the otherhand, you can hardmod your system, in which you would need to open up your wii. You would then need to install a modchip. You can then play downloaded games from DVD, SD card or a hard drive. If your wii is a new model, then you may not be able to hardmod or even softmod it at all.