Hi C., I just happened to have gotten back from San Antonio from our Family vacation and we did go to SeaWorld.
I feel your pain about hotel room expense and here's what I can tell you. Days Inn Hotels are not the absolute best but they are clean and they do have a very good breakfast every morning from 6 - 10 am. I ended up getting two rooms and each had a king bed and a couch that folds out. Now if you call make sure you ask if they have a room with two double beds and a couch(roll-out bed). I would book with the one by the Mall and Alamo right down town. You will be able to walk to Alamo,downtown areas and catch the Riverwalk by going into the Mall (nice cool down). Seaworld was a blast. To be honest you could do SeaWorld for two days...there is so much to do. I've not been to Fiesta Texas but SeaWorld has rides, Water rides, beach wave area. We did not do the Seafair Tour or Penguins close up...but did take in several shows (Shamu Believe), Animal show (trained dogs,cats,pigs), Dolphin/Seal shows...so much to do there I think your boys will have a blast! Downtown.. we did the mirror maze...it's not that big and they can't really get lost, what's good is they can stay in there for quite a while and you can come out and wait and REST! Then there are about 5 places to see (Ripley's Believe it or not, Wax Museum, etc) Some little expensive for so be prepared for that cost I think it was something like all 5 for $47(adults); 2 for $21; 1 for $19.
kids tickets little less depending on age.
All in all I think you all will have a GREAT time!