The absolute easiest seeds to sow has to be the California Poppy. Not only are they gorgeous in the spring and early summer sunshine, they are absolutely PERFECT to sow amongst your tulips and daffodils, as just as those die back, the poppies will rise up and take their place.
The only thing to watch out for? These girls LOVE to self-seed all over your yard! In most places it's a very good thing, but I did have to pull a few newbies out of our raised veggie beds. Other than that though, aren't they marvelous?