Although not the easiest of Shakespeare's sonnets on a first reading, Sonnet 73 is close. The thought behind it is very universal and accessible (a guy getting old wants to make sure people still love him). The structure is also pretty easy to wrap your brain around. There's three quatrains dealing with the same basic theme in different imagery. Then there's a couplet that sums things up and takes them in a new, but totally followable direction. On your second, third, fourth… hundredth or whatever reading, things might get a little more complicated—once you start noticing more and more ways in which the different metaphors are folded inside of one another. By that point, however, you'll already have the basic idea, so uncovering new layers in the poem will be fun, not just difficult.