Unless you already know what to get them, go for gift certificates, they love those.
In the future, spend time with them and communicate with them throughout the year with gifts in mind, and you will have many hints and clues about what they want or like.
Then, by the time Christmas rolls around, you will have a good idea what to buy them, and often it will be something they really liked but forgot all about, so it will be a true thoughtful present, something that not only surprises them but pleases them as well because you took the time to think about them.
I have massive experience with this. You don't just want to get something from a list or give a gift certificate most of the time, that is just like being a personal shopper and is not in keeping with the spirit of Christmas or gift giving.
Also, you don't want to surprise them with something they hate or something you think they should like.
Anything too practical is heavy handed. Gift giving is a light activity. Make it something they might not have bought for themselves and appreciate.
Just keep Christmas in the back of your mind all year, and when you are hanging out with them, or walking around town, see what makes them light up or get excited, and remember it for later.
This is no trial, is actually fun to do, and it makes gift giving special for everyone because your gift will show them that you really understand them and care about them, and as long as that is true, it doesn't even have to be expensive, it only has to show thought.
Like if you are with a preteen girl who looks wistfully at a nail salon as she walks by, get her a gift certificate for that place for next year. She won't even remember the moment, but you will, and she will appreciate it and feel grown up all at the same time.
Years back I had a family member who loved music and mp3s and sharing them and burning CDs. I got her an external hard drive which few really knew about at the time. She opened the present and was disappointed. No buttons to press, no trays that open or closed. She had no idea what the point was.
Two years down the road though she thanked me out of the blue, saying it was one of the best presents she ever got. It was a time released present. She didn't even know she wanted it, but I did because I knew what was important to her.
I'm telling you, this is how to do it.
I was the main present buyer in my extended family when I was married, sometimes buying 100 presents a year. Just a knack.
Women always think they are so sensitive and caring. If that is the case, why were they all clueless at the holidays and had to ask me what to buy for who?
Not that hard to do, just take the whole year to be on the lookout and there is no stress, no pressure and you make everyone happy.