Trouble picking a major
I believe that double majors are for the most part a waste of your valuable time. This is coming from someone who did a double major. Man do I wish I could go back in time and take out some of the boring classes I took just to satisfy the beuracratic requirements of my second major for more interesting graduate and upper level classes of my choosing (some of which actually being in that major's field) and more time spent on extracurricular research/internship projects. 

My advice. Outside of the first major, take the classes that interest you. If that allows you to get a second major or minor, great! If you have to take a bunch of basic classes that don't interest you just to satisfy the requirements of the second major, don't do it!

Usually you can use a personal statement or CV to show people that you are the right one for the job or graduate program you want. Degrees in Applied Physics and Computational Physics are close enough that it shouldn't matter if you do go the double major route (I'd lean towards applied Physics given your interests though, comp physics doesn't say much more than your CS degree already does). However, in my opinion if you are set on C.S. and are just looking to do something related to the physical sciences/engineering, I would get a C.S. degree and then take a bunch of physics classes that interest you and build a portfolio of physics related projects you have worked on. Maybe apply to physics REUs that sound exciting. Many schools have undergrad research projects on electric, hydrogen and solar vehicles, so look for those if thats what excites you, and if your university doesn't have them look for summer opportunities that do.

I'd say for at least 90% of students, a double major is a bad idea. I'd discourage it!

Note there are some cases where a double major may be a good idea. Say your first major is History and its your love and passion, but midway through school you realize that you likely won't get a good job with that major. Getting a second major may be important in this situation because the fields are almost complete opposites, but if the majors are somewhat related I still hold on to my general opinion.