Goats Beard will be about 3 feet high. Nice blooms and interesting foliage.
Columbine and astilbe would work, too.

For perennials, I might try coleus - nice color. Likewise, caladiums.

I love hosta, and so many beautiful varieties are available, they are always
a good choice.

Last year we had our deck rebuilt (added real footings) so this year I
want to plant something that will fill out and cover up the space
between the ground and the decking cause it just plain looks bad the way
it is.
deck is on the north side of the house, soil is quite sandy (live on a
lake) well shaded most of the day.