I want to occasionally switch my internet connection from my wired router directly to my cable modem. Is there a piece of equipment that will accomplish this easily?

No, any time you change the hardware hooked up to a cable modem the modem needs to be reset, and because ethernet has to be a point-to-point connection there aren't "switches" like you see with serial or USB devices.

Many, if not most, cable modems also have a USB connection. If you really needed to have the direct connection available all the time it might allow you to leave both the USB and Ethernet at the same time and just disable the network connection you're not using in windows.

If not, you have to disconnect the ethernet cable from the modem-router and connect it to the modem-PC and reset the modem (every cable modem I've ever used required a reset, but there may be some modems on other ISPs that don't require it). In a larger network environment this could be done with a patch panel, but for you I'm sure just using the port on the modem is more practical.