Anything cheap on ebay, motors sensors, etc is always good to play with, some things may not be quality to actually depend on but for playing around with you cant go wrong
one in particular is the nrf24l01, it goes about 3.50$ a piece and with two you can have two arduini communicating wirelessly,
Also 1602 lcds are good, ive never gotten a faulty one yet amd those are cheap and a good addition to any project
ds18b20 are easy to use temp sensors that can be strung together with 2or 3 wires for accurate temperature readings, also not too bad on ebay
and ofcourse various general purpose transistors and mosfets as well as some high power leds are aleays fun