Fort San Jacinto Historic Site is a ruin of a defensive position located overlooking the Ship Channel.  To get there, just take Seawall Blvd all the way to the end where you will find a parking lot with great views of the shipping lanes.  There is an old gun emplacement right there in the parking area that you can climb on.  For the brave, you can head down to the shore and scramble across the rocks for a long ways in the Galveston sunshine.  Eventually, you will run across the ruins.  I was hoping for cool graffiti, but found a lot of scary stuff instead.  There were a few pieces I liked, but this is probably not a nice picnic spot despite the awesome views from on top of the old fort.