Here is another thought -- there is a hike at Rainier called Comet Falls. In early July, at around noon, the sun angle causes a really unusual phenomenon -- a circular rainbow! It is a pretty tough hike, but my 8 year old and 5 year old boys did it (30 years ago), with some coaxing.
When you get to the base of the falls, you walk as close to the water as you dare -- the rocks are slippery. You will get soaking wet (which is ok, since it can be pretty hot). But the spray rises up around you, surrounding you with a bright circular rainbow. The rainbow is not there in September, we discovered.
Bring a plastic bag to put your camera into. My wife, who is a rainbow and waterfall fanatic, said that this was one of the most amazing experiences of her life, which is quite a strong statement.
