I think it's perfectly fine to buy a gift that isn't on the registry. But I think it's very important that a gift giver buy something that they genuinely think the recipient will like. Not just something that that the giver thinks the recipient ought to like.
And, that means that if the giver doesn't know whether the recipient would appreciate the towels or the tent or the rock climbing card or the blender, then they should ask. Ask someone who does know the couple well enough. Because just because they're planning an elaborate honeymoon to, say Grenada, doesn't mean that they'd want to go skiing or camping.
I mostly agree with this. Certainly I think we'd all agree that gifts like religious objects or books about a religion that the couple don't adhere to, or a subscription to a magazine that espouses views with which they disagree, or BBQ tools for vegetarians, and so forth, are completely inappropriate.
But I recall from my own wedding (back when the surface of the earth was starting to cool) that often our parents' generation had better ideas than we would have ourselves. They knew better than we did what we would need in years to come, when we began to take over hosting holidays and such, and what kinds of things break or wear out more so that you need more than you think you do.
Our peers (we were in our mid-20s) tended to give us things like a bed tray, a subscription to the NY Review of Books, and a picnic basket. Our parents' peers gave us more "boring" stuff -- how would we EVER use all those platters and pitchers and tablecloths?
Now, please understand, we appreciated and loved and used all these things, from both groups. But almost 32 years later, I can tell you which things we ultimately have really depended on and needed, even if we didn't touch them for the first few years (yep, EVERY ONE of those platters and pitchers and tablecloths), and which were a fun novelty but after a while were mostly just a storage problem.
I think that the difference is that our peers were thinking in terms of what we would like to have RIGHT NOW, and the older generation was looking down the road several years and chose gifts meant to last a long, long time, even as our lives and interests changed. (There is nothing wrong with either kind of gift, and in fact I think it's nice to have some of both.) A honeymoon fund is probably the ultimate "right now" gift, which might be part of the reason many people don't like to give that, even if they don't find it crass or don't care.
[I saw the same principle in action years later when we had our first baby. Our childless friends gave us those darling newborn size outfits that you're lucky if the kid can even wear twice, and that's assuming the season lines up right. Then other gifts -- from people who had raised kids themselves -- arrived in sizes 2, 3, and even 4. "What are they thinking?!" we wondered. I would never have told them that's what we wanted if they had asked. But they knew better than we did. Again, it was nice to have some of both kinds of gifts, but was really great to have a few cute new outfits ready and waiting each time the kid grew a size, and we certainly got a lot more use out of them.]
I'm guessing that most younger couples getting married would tend, like the young friends in our story, to be more "now-focused." And that's why I don't agree 100% that it's important to buy only what you know that the couple has already expressed an interest in, and if you don't, then you must ask them.
Besides, if you miss the mark and get them something they don't want (e.g. wrong color place mats) or don't need (e.g. a blender when they already have a good one), they can exchange it or give it as a gift to someone who does. (So do avoid, unless you are SURE they will want it, things like monogrammed items, consumables, and artwork).