well if you have friends..hehe.. another wiimote.
um..playing with your grandpa, buy wiiplay(comes with a wiimote) so you kill two birds with one stone.
i just buy a 12 pack of batteries but some poeple prefer the nyko wiimote charger.
if you like shootem fps games then get the wii zapper. not many games use it yet.
maybe a gamecube controller if you wanna play cube games or a calssic wii controller if you want to play the virtual console games that you can buy with wii points in the wii shop channel.
if you want to play online.. a wii lan adaptor.
games.. i recommed mario galaxy to start.
oh and if you have an hdtv get the wii component cable.