Give a faster Mac for not much money
There is no better way to speed up an older Mac than to add a solid state drive (SSD) as a boot volume. It matters not at all which modernish Mac model you're talking about. If it currently uses a spinning mechanical drive, replacing that drive with an SSD will produce jaw-dropping results.
If installing such a thing is beyond your ken, you can find instructions from both iFixit and OWC (and each can sell you a drive).
Note that Yosemite doesn't support TRIM on third-party drives (see the linked article for details) so you'll want to be careful about which SSD you choose.
Also, as an SSD is likely to have less capacity than the drive you're replacing, you should consider storing items found within the Home folder on another volume. And if this Mac holds just one drive, don't fret. In Giving the gift of speed: The SSD upgrade I discuss adding a second drive to certain Macs.
Power to your people
This next suggestion is more or less helpful depending on where the target of your holiday affection lives. Country dwellers with questionable power lines and those subject to weather-related outages will find it the most useful. And that suggestion is that you carefully wrap an uninterrupted power supply (UPS) and shove it under the mistletoe. Both you and it are likely to get a kiss.
For those unfamiliar with these devices, a UPS is essentially a large battery that provides additional power outlets and a measure of surge protection. Should the power suddenly go out, a UPS can keep a computer running for awhile--long enough to save your work and then safely shut down the computer. APC is probably the most well known brand, but other companies make these things too.
When giving the gift of power you'll want to be on hand when it's put to use. And no, not just so you can receive warm thanks. Rather, you should be there to help ensure that the right devices are plugged into it. For example, only key components should pull power from the battery. There's no reason to attach a laser printer to it, for example. Nor should you tax the battery with a nearby TV. Instead, be sure that the computer's power cord is attached as well as any peripherals that the computer absolutely requires--a monitor if it's attached to a Mac mini, for instance. You could also attach the broadband modem and router to it so that your family can continue to use the Internet until such time that the battery drains (helpful for checking a power company's website to learn when power might be restored).
The backup strategy
As often as you and I harp on the need for some kind of backup plan, too few people have one in place. And if your nearest and dearest haven't yet thought about implementing such a thing, no amount of yammering on your part is going to make them do it now. So make it your job.
Doing this is as easy as purchasing an external hard drive that's about twice the capacity of their Mac's drive, attaching it to their computer, and configuring Time Machine to use it. Slap a sticky note to it that reads "DANGER! HIGH VOLTAGE!" and warn them that terrible things will result if they detach or switch off that drive. If they have a laptop and routinely move it from place to place you can temper your instructions so that they're less alarming.
If that seems like too much of a burden for them, enroll their Mac in an online service that automatically backs up their data. There are a wide variety of such services including Backblaze and CrashPlan. When considering this strategy be sure the person's broadband plan and connection can support it. If they have terrible bandwidth or a very restrictive data cap, online backup may be more hindrance than help.
The password strategy
As we all conduct greater amounts of business on the Internet (and trust more of our personal data to it), having secure and varied passwords matters. A lot. Surely you're using the password features built into OS X, but your friends and family may not be. You can be helpful in this regard in a variety of ways. Start with The Talk, which includes the following rules:
Once you've given The Talk and been replied to with nodded assent, ask that your friend write down every important password he has. Pull out your iPhone and take a picture of that list and store the image in a safe place where you can find it. That way, when your beloved calls six months from now and tearfully cries that he can't remember his Apple ID and password, you can provide him with it.
If the recipient of your gift is up to it, show him how to add passwords to OS X's keychain and recover them from Safari's Passwords preference. If his needs are greater than what OS X can provide, buy him a copy of 1Password and show him how to use it.
Keep in touch
You've earned that extra slice of plum pudding, but if you'd like to make a down payment on next year's portion, you'll go the extra mile by making your help available year-round. But be smart about it.
Not smart is giving Uncle Bud your phone number and telling him that he can call day or night with any small problem he encounters. Not only will you be contacted at all hours of the day, but there's nothing as frustrating as trying to troubleshoot problems when you can't see what the computer (and Uncle Bud) are doing.
Smart is, instead, configuring his Mac so that you can control it remotely. You can do this via Messages if you're each using an AIM, Jabber, or Google Talk account in Messages. If you're a little hazy on the details, Topher Kessler's Away from home Here's how to access your Mac remotely will help.
That way, when your favorite uncle gets stuck, he can call, you can each log into Messages, and then--after a stern "Don't touch the mouse/trackpad or your computer will catch on fire!"--you can set about untangling the mess he's created.
And to all a good night
Some of you may view this as the cheapskate's way to avoid the trouble and expense of getting a "real" gift. Humbug! Anyone can swing by Gifty Mart and pick up half a dozen radioactive reindeer. It takes far more effort to provide another person help in a way that's truly meaningful. Perhaps it won't pay off today, or even tomorrow. But when you pull your cousin's data from the edge of oblivion next Arbor Day, she'll know who the real Santa is.