Hi Deb,
No lenses: the S900(0) has a fixed lens.
Filters: UV filter and CP filter (Circular Polarizer). Most other effects can be achieved much better, much more precise, and much more conveniently in post production (editing). So save yourself money and don't get filters for them.
The pop-up flash is not called a peanut flash for nothing. It is useful for fill-flash and relative close-ups: within 4 meters or 13 feet max.
So you could get yourself an external flashgun with (a lot!) more power, to stick in the hotshoe. Or – and that's better – also get a cord/wire for off-camera, remote flash. For more creative and distributed lighting and less blow-outs.
The S900's manual is your friend! Read it carefully.
Have fun!