Depends on what accessories you have around the house already (keyboard, mouse, protoboard, USB wifi if you go that route). I've bought Pis as single boards and as kits with accessories. I did have to get a hdmi to dvi cable to work with the monitor I had. If i had nothing at all and wanted to experiment I would probably get the kit that comes with the protoboard, and the little electronic bits like thermometer, infrared etc etc. I also highly recommened this book : http://www.amazon.com/Raspberry-Pi-Hacks-Inexpensive-Computer/dp/1449362346/ref=sr_1_1?s=books&ie=UTF8&qid=1420940965&sr=1-1&keywords=Raspberry+Pi+Hacks
You might check Raspberry Pi Hacks for some cool ideas. I found it a good read even though I wouldn't be doing all of the hacks.