A really great gift in my opinion is always a watch because men can't get enough of them. I find that they usually have one or two investment watches alongside more affordable 'every day' styles like the classic ones sold at Daniel Wellington. This face is actually unisex too so Adrian and I share it!
I personally love buying clothes for guys and depending on the budget, I go all out. I think studying menswear gave me quite a good eye when it comes to dressing men, but you can never go wrong with an investment jacket or even a quality shirt for work or formal occasions.
Other easy options include toiletries and boxers/socks. Its like the one present that is useful and long lasting. Personally, I'd love tons of new lingerie every Christmas but I don't think it works the same for girls!
If you're dealing with a technology lover then you could always opt for something like a portable splash-resistant speaker from Libratone, some wireless headphones or even a GoPro. Obviously what you get depends on budget but sometimes the accessories are nicer. Does he have a fancy DSLR? Maybe get a camera case or camera strap.. the same goes with phones because theres a wide range of cases out there too. Alternatively - go for something travel focused like a leather passport cover and luggage tag set like this one from Vida Vida. It always goes down a treat with someone who either travels a lot for fun or work!
To save you searching the internet for hours, below is a little gift guide so you can shop for your loved one without needing to spend hours on different websites! :)