So my boyfriend loves to tinker with his Arduino and I was reading up on some fun stuff to do with it and I thought he'd enjoy 're-purposing' old electronic stuff. I was planning on hitting up garage sales/thrift stores for things he (or I) could salvage into something fun.
So my question for the forum is:
Anything suggestions of useful kinds of parts/things to look for?
Thanks!
P.S. Sorry if this isn't in the right part of the forum for this kind of post...
Pretty broad question 
I'd enjoy anything that has a definite shape (i.e., is an enclosure, in some kind of box), has some types of buttons, and has a classic feel to it.
It's almost impossible to go wrong here. You can get 10 things, let him pick 1 or 2 that he likes, and re-sell the rest
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The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons
Remote controlled cars/trucks for motors and power transistors.
Printers for stepper motors.
Toys with LED displays (LCD and Electroluminescent displays are hard to connect to).
Toy drum sets will have a bunch of piezoelectric disks to use as knock sensors.
Electronics from the 70's, 80's, and 90's will have more harvestable through-hole parts and fewer custom chips and surface-mount parts.
johnwasser:
Remote controlled cars/trucks for motors and power transistors.
I second this - these make great robot chassis if you BF is into that kind of thing; just make sure nothing is broken, and that the motors seem to rotate fine; spin the rear wheels and make sure you don't hear or feel any weird or abnormal grinding, also make sure the steering seems in good order (wiggle the wheels, let them go - they should return to center). If you can find an old R/C or wired-remote controlled tank (once again, check the treads/gearmotors for weird sounds, no binding, etc) - even better (they are difficult to find, though).
Another alternative is a Robosapien, or a RoboQuad toy.
johnwasser:
Printers for stepper motors.
Definitely - also a great source of bearings and slide rods for custom CNC machines or robots; another great source for this are old scanners. Stay away from copy machines, though, unless they have removable toner cartridges - you don't want a toner mess! But they are also a great source for similar parts (same with fax machines).
johnwasser:
Electronics from the 70's, 80's, and 90's will have more harvestable through-hole parts and fewer custom chips and surface-mount parts.
Be sure if you buy anything from the 70s or 80s that the device isn't considered an "antique" or "collectible" item; some old audio amplifiers from the era (and turntables, and tape decks, and you name it), as well as some computer components (and toys - speak n' spell, big trak, etc) - are -very- collectible; I've found at thrift stores and yard sales items going for nothing (I once bought an Atari Video Music for a buck!). The last thing you want is to tear something apart, and find out later that it was worth a lot of money (especially if the amount you spent on it was little to nothing).
This goes for computers, too: People are willing pay some big money for certain old machines (and old peripherals). It's becoming really difficult to find old 5.25" floppy drives (especially the low density ones - which usually had nice stepper motors in them) - people pay a lot of money for them today. And have you seen the prices people are asking for 386 motherboards? Especially the AMD 386 DX40 (that's a sweet ride there)... 486 motherboards are also going up in value. Apple IIe, C=64, Atari... If you ever run across an Apple Lisa you've hit the motherload...
Count the number of buttons, knobs and switches. In the old days when there was plenty of war surplus about the dealers used to value it at "a bob a knob", a bob being a unit of currency in pre 1970s Britain.
Bluebell,
Do you have any idea what kind of projects he's building ?
That would probably make the question a little less broad.
I don't know whether he already has one, if he doesn't a Digital Volt Meter is something
any electronics-freak can use. A few $ more might... be an old oscilloscope. An adjustable power-supply
from 0-30 volts with a bit of power is also something that's pretty universal.
If you could try to hear him out a little without him noticing too much we may get an idea
of what he probably really would like. Just start asking a little, tell the results here, we may be
able to provide you the right questions to dive deeper in his interests and with a bit of luck
you'll have a great surprise for him.
Hopefully we'll also be able to help you a little with what you should spend maximum for items of choice since some people have no problems asking 4+ times the price an item's worth.
Last but not least, I hope my girlfriend has started posting on forums and uses the name Bluebell 
Haha thanks everyone! I know it's sort of (too?) broad a question but I really like a lot of the suggestions (especially a stepper motor...) For an example of things he's done...well, mostly he's automated his homebrew process. But some of the most fun he's had has been with a pneumatic switch he salvaged from an old paintball gun...Most recently he's used his arduino to make a level control for a cooling jacket for his fermenter. But he's the kind that likes to tinker with whatever's on hand (like pretty much everyone else here I'm beginning to realize) so I thought if he had more random things just laying around...
Anyway, thanks for all the suggestions!
Simpson_Jr:
A few $ more might... be an old oscilloscope. An adjustable power-supply from 0-30 volts with a bit of power is also something that's pretty universal.
Any "old oscilloscope" in the likely budget mentioned (I am assuming under $100.00 USD) is likely to either be non-functional, grossly uncalibrated, or have such low-bandwidth (sub-1 MHz) as to be nearly worthless for anything more than a collectors item. As a collectors item, a very old scope can be a fun thing to have around as a decoration or similar, but not likely for day-to-day usage. Anything that would be worthwhile (working, somewhat calibrated, decent bandwidth) is going to cost more that $100.00 USD - and you would want to know what you are doing to test the scope before purchase (if you are purchasing a tool, rather than a collectible), to know if it worked right and you weren't throwing money away.
With that in mind, a decent used analog scope (talking something like an HP, Fluke, or Tek), say dual-channel, 50 MHz or better - will run between $150-200.00 USD, maybe a bit more depending on "extras" (if it comes with probes, has been calibrated recently, has docs, a stand, etc). I once got a dual-channel 100 MHz Fluke CombiScope (I think that's right) for $200.00, but that was a very lucky find on my part, I think.
As far as a power supply is concerned - depending on what you are looking for, they can be expensive too, even used. A good single output adjustable supply shouldn't be too terribly priced. I always like looking for dual output fully adjustable supplies, but they aren't found often, even at garage sales. If you want to have any luck, go to retirement communities which might have a bunch of older hams in residence having yard sales (that, or attend a ham fest swap meet).
The epson and HP "deskjet" style printers also have rottery encoders in them, and some nice decent power powersuplys.
(typically 32 and 14volts in 1 package)