I continually hear people bad talking the Alamo. (It's so small, It isn't interesting, etc.) If you appreciate history, you will enjoy it. The fact that it isn't interpreted like a Disney Theme park ride is a good thing in my mind. The fact that it is small makes the efforts of those defending the mission all the more remarkable.
Be sure to travel the mission trail and visit the other missions. We used the rent-a-bike kiosks and biked all along the river to the several missions. It made for a great day of history, exercise, site seeing and adventure.
The River walk is OK, but also get up on the main street level and experience some of the shops and history of the area there as well. There is plenty to see, and you can spend several days doing it.