Plants that have storage organs do well with episodic watering. These
include bulbs: daffodils, tulips, some lilies, etc. You could plant a
carefully selected set of bulbs that bloom in sequence through summer
Can you swap out the tubs? If so then two other plants that come to
mind are irises and daylilies. They tend to crowd out other plants,
though, so you would want them in tubs without other kinds of plants.
Many irises and daylilies bloom just once and are finished, but there
are so-called reblooming varieties that last longer.
If Skimmia works then you are somewhere that is warm temperate.
Xeriscape plants that would work include some of the smaller, hardier
cacti. Else small succulents. Both can look really fabulous in
shallow tubs with nicely sorted river gravel or lava rocks. Visit
public alpine gardens and rock gardens to get ideas.