If watering is the only issue, try mixing some of those water-absorbing crystals in with the soil when you plant.
They soak up lots of water and release it slowly so your plants can go without wtering for a longer period of time. Can't remember the name of them, but almost any garden supply center will carry them. My personal preferences would be pansies for the north side and maybe portulaca (which likes a drier soil) for the east, but adding those water crystals will really increase your options.

Thanks Mainerose, By starting early I will start thinking about what I want planted in those boxes and what I need to bring with me. One year at home, I made the mistake of planting flowers that needed water at least once a day. They kept my mother busy, that summer. I like your idea of pansies, they might tolerate cooler,long term weather.