Ditto what dinahmoe said - sounds like he really needs a hydration pack. Camelbacks are good, except the bladder material makes the water taste funny. I like to use the Platypus bladder inside a Camelback pack.
Another idea: you said he has a GPS, but does he have a wrist-mount GPS and heart rate monitor? Timex also makes a similar product but it’s a bit more expensive.
A Klein bottle, as mentioned above by sleestak, is a very cool gift for a geek. Even better, in my geekish opinion, is a more practical item, the Klein Stein drinking mug. I’ve been wanting one of these for ages, but so far nobody has taken the hint.
From what I know of the gentlman in question, a suture kit might be more in order. 
Admit it, counselor…you used this as an excuse to go to REI to look at headlamps for yourself.
What about some kind of combination daypack/camera bag? Man can never have too many gadgety bags.
Black Bush Irish whiskey. Oh, sorry, that’s me. Never mind.
Stranger
Get him a chess program - like Fritz 9, or HIARCS 10.
These can be wonderfully geeky programs. You can get them for ~$47 each plus S+H at
http://chesscentral.com/, or
http://chessbaseusa.sectorlink.org/DesktopDefault.aspx
And at 66, he’s not too old for chess.
I called my sister (aka, The Good Daughter) to ask what I should get dad. She wasn’t home, so instead I spoke with my dad’s hiking partner (aka, my BIL). Who said, and I quote (aka, paraphrase loosely), “No, your dad doesn’t have a camelbak, and packs his backpack full of bottles of water that he doesn’t drink, so my pack is like 30 or 40 pounds, what with the sunscreen and the bug stuff that your dad also refuses to bring, and your dad is like a mountain goat, but I can take him on the flat areas.”
There was some other stuff in there, but that’s the highlight. I think the other stuff that I left out probably would make that statement make sense, but so it goes.
I have now ordered a camelbak for my dad. (Stranger, I agree that while a suture kit would be practical for my dad, I don’t want to encourage him to lop off any more appendages (see, e.g., nut doesn’t fall far from tree) and then post pictures on his webpage for the family’s edification. dinahmoe, does this still sound like your dad? If so, you must be one of my siblings, in which case I hope you’re having fun today and I’ll see you at the birthday party. ;))
:eek: That is, generally, a flat out lie, for the most part. Besides, my BIL (who uses his for reading) says he’ll get me one for my birthday.
“…a flat out lie, for the most part,”? You must have gone to a really fancy law school to reason like that.
Shucks, I was going to get you a headlamp for your birthday. Now I’ll have to figure out something else. Maybe a subscription to “Semantically Tortured English Quarterly” instead? 
Stranger
That’s right. And I have the debt to prove it… 
Thanks to the beauty of internet shopping, as long as I remember to buy a card and some wrapping paper, dad should have a good birthday this year. Maybe I’ll get to be The Good Daughter for once!
You know what they say about heightened expectations.
But you’re a good kid anyway; I don’t care what everybody else says about you.
Stranger
Tell us a bit about him. Job? hobbies? Likes? Dislikes? Faveourite food, movies, places, people, etc.
My dad was a programmer. Worked on a variety of things, including a stint at JPL. He remembers when computers were the size of small buildings, and was amazed as they got smaller and smaller. When I was young, he worked two jobs at times, or very long hours at one job (sometimes commuting hundreds of miles), to support the family. In the winter, he would build a snowman outside the living room window, so that we’d see it when we came downstairs to get ready for school. He shaved his beard to teach us to blow bubbles with bubble gum.
He retired several years ago, having successfully launched all his children (and having invested wisely). Now, he maintains a family webpage and a webpage for my mom’s classes. He plans travel for them around my mom’s school vacations, and in his spare time remodels the house (slicing off the random chunk of flesh) and studies different programming languages.
He is bossy, and a perfectionist. When I was six or seven, he tried to teach me to make my bed so that he could bounce a quarter off it, the way they did when he was in the army. He’s not a picky eater – as long as he’s fed at regular meal times (8, 12, 6), he’ll eat anything. If you don’t feed him, he gets grumpy. He’ll go to the movies if he’s got a coupon, but otherwise, he can take or leave them. He loves my mom, and he loves his children. It’s sometimes hard for him to know how to show it, but he’s getting better. His favorite thing is new people; sometimes I think the reason I bring so many of my friends home to meet my parents is that it’s such a joy for my dad to have new people to tell his stories to.
Does he enjoy music? You could take him to a jazz club, symphony, whatever. I did this with my Dad one year and it was tons of fun. Plus, the added bonus of spending time together.
Does your local paper have a “happy events” section? ours does on the weekend - big double page where people can put in pix of happy couple getting engaged, happy couples at their wedding, happy singles leaving divorce court, birthdays, anniversaries, etc.
Dad turned 80 this week, so we all sprung for an advert with his picture wishing him a happy birthday.
He sings in the church choir. At the end of the service the priest does the routine announcements for upcoming parish events. Yesterday, the priest said, “I’d like everyone to stand up and sing ‘Happy Birthday’ for Mr. Piper Sr. who just turned 80.” So the whole congregation sang Happy Birthday.
He was pleased and surprised.