What is one thing that you think engineering students should develop/begin to develop during their time in undergrad?

Those are all potentially useful things to know, but I wouldn't get too stressed out about not having time to dive into them as an undergrad. I too had a packed schedule (too many majors), and what I realized after graduating is that I would never again have a good opportunity to do something completely outside the STEM mainstream just for fun. I'm not sure it would have been better to spend an extra year in college, but I'm also not sure it wouldn't have been.

At any rate, while getting a solid education in your major field is clearly important, realize that there will be time after you graduate to pick up additional skills and to learn about new things--both on your own time and on the job. If you're particularly passionate or curious about something and there's a relevant course available, that's probably something you should consider taking. If there's an interesting summer (or off-season) internship somewhere--and keep in mind that such things exist at places like government laboratories (Fermilab, Argonne, LLNL, Sandia, Brookhaven, LBL) and NASA (Ames, Johnson, Kennedy, Lewis, Dryden/Armstrong, ...) as well as tech companies--be sure to apply for one or more of those: the experience will be invaluable in terms of understanding whether that's the kind of work you truly want to do, and each one of them amounts to a multi-month interview and may very well lead to a job offer as soon as you graduate.

As to the specifics, public speaking is something you can pick up later, both by watching others (in person and in videos) and by doing it (for your team first, then larger groups as you gain experience). "Language" is ambiguous; if you mean human language, it's definitely useful to know more than one, but they usually take years of study (or months of immersion somewhere, which can be intensely uncomfortable), and whether you'll actually use the one you studied is anybody's guess. (I didn't.) If you mean computer languages, at least one is pretty much required for any engineering field; for software engineering, more than one is useful, but after one or two, you'll find almost all of them are pretty easy to pick up. CAD seems quite specialized to me, but I suspect it's widely used in mechanical, civil, aerospace, architectural, and perhaps electrical engineering; it simply doesn't show up in software engineering, and almost certainly not in bioengineering (unless you're 3D-printing biological tissues or something).

As for "something completely different," that's kind of what I was getting at in the first paragraph--if there's something available that catches your eye, now is a great time to give it a shot. I took a scuba-diving course, for example (lots of fun), as well as philosophy (not as fun as I expected) and "computers in the arts" (very interesting but also not what I expected). On my own I've played with compression software, computer graphics, crypto software, and virtual worlds and 3D modeling, all of which have been a blast. I mostly don't use those things in my day-to-day job, but I absolutely don't regret spending the time on them, and being able to discuss them intelligently in interviews has probably helped me get some jobs.

I think it is definitely an engineer 'thing' to want to learn more and employers know this. So they send you on training courses during your employment.

I wouldnt worry too much about this, as Greg says, you have plenty of time to learn these extra subjects. But anything that interests you now, go for it. It will look good on your CV and as well as the experience, it show employers you are eager to learn a variety of subjects.

Sally

I'd recommend that you become involved with professional organizations (IEEE, SWE, etc.). These organizations will be there to support you during and after college and they're great ways to network, learn trends within the field, and develop those other "soft skills" that are pertinent to life outside of school (including speaking, presenting, etc.). I wouldn't spend time in a college class if it's something that could be worked on outside of class (such as with organization activities/events, self-help books, or volunteer work). Learning a new language would be (generally) helpful in any career, but like Greg pointed out - it takes time to master! I would recommend dabbling in your interests to become more well rounded and show your desire to constantly better yourself by learning new things in addition to being actively involved (hold an office, plan events, etc) in a professional organization.

There could n number of things which you should do during your engineering.
1) Learn about your subject- it is extremely necessary to know the intricacies of your stream.
2) In case you believe you have chosen a wrong stream, you must explore other streams to find area of interest. This is very crucial as your whole life depends depends majorly on this decision
3) Do work on publishing some technical papers, attend conferences, participate in intracollege events etx. Such stuff gives you extra mile as compared to your peers and equips with practical knowledge which is often missing in engineering education.
4) Develop business understanding along with technical knowledge. Business and technology should go hand in hand.
5) Don't forget the fun. These are the best years of your life. Enjoy them.