Ok this is what you do. You know how there are the sand slide things found around the desert area? Well put two pokemon in the daycare and go to one of these sandslides. Now tape the up button on your gameboy while in line with the sandslide and tada! Your pokemon will grow levels very fast now while you do something else.
Note: It would be a good idea to have a charger on your gameboy because this will suck the battery dry. Also be sure to have lots of money to take them out of the daycare.