Each time I have travel, I ask that same question. Usually I take my tripod. Earlier this year I purchased a light weigh, four section carbon fiber tripod (about 21" with out head) and pack it in my check bag. I went to Yosemite this year and was glad I took the tripod.
Simply put, a monopod will not replace a tripod. Go with the tripod. If you are overly concerned about bulk and/or weight, I recommend that you consider a good (notice the word "good") table top tripod and good (again) head.
If I had a choice between my tabletop head and legs or a monopod, I would go for the little tripod. Tabletop pod can be mounted on virtually anything that does not move. Like a church pew, a column, a railing, a parked car, a tree, a big rock and on and on. There is often (but not always) something to prop the tabletop on. If all else fails, depending on which legs you have, you can even use the tabletop braced on your chest to gain support. (yes - a "chestpod" really works - especially if you can get a decent shutter speeds. You can often gain several stops or more by bracing it on your chest).
My tabletop pod is the ridiculously overpriced (but NOT unaffordable) Leica.
You will not believe how much weight this combo holds. I have shot with a D2X + SB800 and 70-200/2.8 and it holds the gear rocksteady. Uncanny!
Lately though, in an attempt to lighten the load and reduce the size, I found that the RRS BH25 is a terrific little head to use on the Leica legs. Shorter, lighter, less bulky and most importantly less expensive, yet again with terrific holding power.
If you find you can't (or do not want to) lug the tripod, this tabletop is a terrific idea. I actually carry it with me even when I do have the tripod available for situations where regular tripods are not allowed (like churches, museums etc.) Never had anyone challenge me while using the little rig in a tripod free zone. (includes my home town of NYC where the police will often break chops if you try to set up a full legset on a street, but couldn't care less about the tabletop. Believe it or not, the Park Police at the Statue of Liberty have been prohibiting tripods, but not the tabletop.)
Another really useful gadget is The Pod: essentially a small bean bag with an integral 1/4" post that connects to the camera or lens collar foot. I carry one in addition to my lightweight Induro tripod. It is very handy when there is some appropriate object to brace on. I used it on ship railings on a recent cruise and it did a wonderful job of filtering out the vibrations from the ship. I've also used in on car windows and fence railings. It is hard to imagine a more useful tool for the price, about $20 at a local camera shop.