Well of course you cannot miss the missions. There are plenty of photo ops there. The river walk is just a big open air mall, so dont even waste your time there unless you want to eat and drink. It is excellent for that
In Houston is kind of though, Galveston would be a good bet early in the morning. Just whatever you do, please, please dont take a pic of the sky line.........
Ok, let me see, for store fronts and weird stuff the Montrose area is the artsy place to find some good ops.
If you actually feel like doing more seeing than taking pictures I recommend you go to the John Cleary Gallery in Colquit. There you will find many photographs by all those photographers you have heard, and the owner is very nice so if you ask he will let you see what he has that is special. Also dont forget the Houston Museum of Fine Arts, They have a wonderful photo collection and a very nice painting collection also.