It's been a while since I've been on this forum, let alone written anything, but I think I'm qualified to chime in on your dilemma. I majored in civil engineering at Texas A&M University from 2005-2010 (5 years, no summer classes). I had this same question during my freshman year in whether or not I should march. To put it frankly, getting an internship as an underclassmen is HARD and dang near impossible in most cases. Companies will pass you up because they're more willing to help the guy/gal about to get out of school. That being said, I say go ahead and march as long as you can. You have over 40 years of work ahead of you, with only a few to march. After applying for dozens of internships and getting little to no responses, I ended up marching with a top-3 Division III corps, and a top-5 Division I corps in 2006 and 2007 respectively. I was the only engineering major in the D-III corps (of around 75) and one of maybe 3-4 in the D-I corps. Everyone else were primarily music majors.
In 2008, my age-out year, aside from my body took a toll from marching with a contra and from other previous physical activities, I had lost that spark to be in drum corps. I felt I got what I was looking for in DCI. I marched with my dream drum corps, and needed no more to feel satisfied with my experiences. I figured out pretty quickly that some people stay in drum corps because they don't have a lot to aspire to outside of music. In my case, I came to a crossroad in my life. I didn't need drum corps anymore. Marching was no longer a major driving force in my life, and I wanted to start focusing on my career and start building my resume. Not sure if I wanted to make the decision official, I went to one camp in the fall of 2007, and although I put my best foot forward in preparing and auditioning, I hated it. I remembered how tired I was of all the repetition, the long grueling rehearsals, and to a certain extent, the politics that can surround and taint a drum corps. However, while I was there, many rookies and new folks attended the camp with the same excitement and optimism I once had. Pretty quickly, I realized that DCI needs those kinds of kids-not someone doing it just to occupy their summer, so I quickly dropped my name from the running a few days after camp ended. I thought of it as passing the torch to a new generation so to speak. Throughout the rest of college, I still kept playing my instrument with concert and basketball bands. To this day, I still volunteer with my old corps, and go to 2-3 shows every year and proudly wear my jacket, even in the 100 degree Texas heat.
That being said, this was just my experience, and you could have a completely different one. The point I'm trying to make is that I encourage you to do both DCI and engineering. It's very possible. I have no regrets in what I did, and feel very blessed to be in the position I'm in now. Companies like to find people with personalities and interests outside of work and school. If you're serious about becoming a civil engineer though, be sure put focus into finding an internship or co-op at some point that is of your specific field of interest. They can be difficult to find, but they're out there. Aside from one summer internship, I was able to get another job at a local civil firm that allowed me to work part-time throughout the school year and full time during the summer. I wasn't even looking for the job so much as it was just being at the right place at the right time. It boils down to never knowing what's around the corner. As far as doing international stuff, study abroad, or whatever- as cool as they sound, they don't buy a lot of stock for you unless you're specifically looking to work out of the country. If nothing else, get as familiar as you can with Civil3D software, and Excel.
The nice thing about civil engineering is that there will always be a need for them. This isn't like getting some useless degree that qualifies you to work at Dillards, or another engineering field where you could be fired in an instant if the economy takes a down turn. Civil engineers aren't losing jobs that are going to China or India, and as long as people are still breathing, we'll always need water pipes, toilets, roads, drainage, etc. Like I said before, getting a job as a young student is very hard. Enjoy yourself and march while you still have the passion, time, and money. Trust me, there are plenty of people who do get a civil degree who never had a job in college. Having a 4.0 sure helps, but being more well-rounded helps even more. I finished with a 2.95 GPA, and no company I was interested in cared the slightest bit. The ones who did, I wouldn't want to work at anyway. Remember, you're only 18 years old, and you have your whole life still ahead of you. Whatever you do, work hard, play hard, be honest with yourself, and make the most of it.
Hope this helps, and best of luck making your decisions...