I did a week-long bike tour in April in Mississippi where the sun was intense and temps were in the low 90s every day. I wore Shebeest Shindiggers, which covered my knees and were sufficiently lightweight that I didn't get any hotter than I would have in shorts. Plus I liked the protection from getting burned.

If you want a capri to wear over regular bike shorts, you might take a look at thesefrom Land's End: http://www.landsend.com/pp/WovenBike...&origin=search

They're sized to be loose enough to wear over a pair of bike shorts. I haven't ridden in mine yet, but the fabric is lightweight and the cut is comfortable.

Have fun!