Selecting multiple objects in floor plan view: pick the "Select Objects" pointer icon (or hit the space bar to do the same). Click and drag over the items you want to select. I always get roof details included in the selection, so when only working floor plans, I turn off most or all of the "Roof" options in the Layer Display Options dialog.
Selecting specific items in any view: again with the "Select Objects" pointer, select an object, then hold the Ctrl key (Windows) or Command key (Mac) and select another object.
For the hexagonal island, unless you can find one pre-made in the Library Browser, you would have to use another 3d CAD package (like Sketchup or something) and import it into HD. You might be able to make something that sort of works with the "Shapes" available in Library Browser, and then click the "Make Architectural Block" icon in the status bar at the bottom left ... though architectural blocks may only be available in the Pro version, not sure.