You are looking for an auto top off unit. If you are only gone a few days a 5g reservoir would be adequate for you. I used to use the 5g water change jugs and switch them out, but any food safe container will do it.
The cheaper auto top off units will just operate off of a pair of float switches to activate the top off pump, and then automatically turn it off when the tank has refilled. Make sure you get one that has a pair of float switches.
Their are more spendy ones that use an optical sensor. They are good too. I use the TUNZE one now that used a single float switch but I pair it with a different optical sensor for double protection so I don't flood my office. It sounds complicated but once you see how it works it makes sense and should work great for a long time.
Good Luck