We've been using MacBooks so long now that quite a few are past their three year Apple warranty period and unlike PCs, they are still in remarkably good shape. So rather than throw them away, these seem like things that work well enough to be repaired. Here are the top few that make sense for Macbooks up to 2009:

1. Battery Replacement. Particularly for the older MacBook's, this is easy. Just search Amazon for an OEM replacement from Apple and for about $85 you can have you battery life back. Just make sure to charge them all the way up and all the way down. For the MacBook mid 2008 this would be about$50
2. SSDs. Going solid state does wonders for battery life and also reliability. Again on the original MacBooks this is easy, they are held in by a single screw. Just get a new Samsung 830 or 840 SSD and you are rock and roll. The 840 is just coming but the Samsung 830 is $160 at Amazon for a 256GB drive which is terrific deal and really adds to performance and$130 from Newegg.
3. Keyboards. If you aren't careful you destroy these. Again for $90 you can get a new top deck and keyboard. As an aside, getting an iPearl cover for$20 makes a lot of sense. Includes a piece of plastic for the keyboard too. Things are a little complicated as the keyboard is screwed into the top case and costs $300 to replaced in an unibody MacBook. The other options are to just replace the keyboard by unscrewing lots of stuff or to replace just the bad keys if the whole keyboard isn't bad. So you can buy a keyboard for about$50 and take keys out and replace it or unscrew 50 screws and put it in.

For the original MacBook Air, things are more complicated because it wasn't designed with user accessible parts but people have tried it. There were two flavors the MacBook Air 1,1 which was the original 2008 model and then MacBook Air 2,1 which was late 2008 and 2009.

1. The hard disk on the original MacBook Air was a strange PATA with a special connector and OWC has a replacement for it. The speed difference is dramatic as well. It doesn't cost a super amount \$150 for 120GB and really does extend life. The original SSDs were about twice as fast as the hard disk and this is even bigger as the disk gets fragmented. iFixIt has a great guide that shows how to make it thru this complicated process.
2. The biggest update may be simply providing more memory as these original machines only had 2GB but it seems hard to find something that does this upgrade. I'm wondering if the memory is soldered in.