For floating silk plants, I've used the fall leaf sprays they sell at walmart. You can cut the leaves off of the wire stems, and then hot glue them together into lines or bunches, and they'll usually float flat on the surface. If they won't float then you can take ribbon, sew the ribbon onto a portion of the bundle, and then use clothes pins to attach the ribbon/leaves to the side of the tank, and they'll float at or near to the surface. My guys love them, they all hide under there and blow their bubbles.
Not sure what fishing weights are made off...I haven't used them myself, but I saw them recommended for weighing down home-made sponge filters. The river rock and sealant sounds like a safer idea though! :)
Oh, and thanks for the adhesive information!! :)