The nicest place that both my wife and I have visited here in the states has to be San Antonio with its Riverwalk and the Alamo.
Both are not to be missed. The Alamo is very serene, peaceful, and just an amazing place to walk around and take in its history and beautifully landscaped gardens.
When you walk into the Alamo you are asked to be quiet, not to take photos inside, and be respectful of its historic significance.It is after all a shrine to those who gave their lives for freedom.
The Alamo is a great place to just take in a little history and relax for a couple of hours.