I think Kevin36's idea is a solid one. There have been several people who have posted on this forum with below a 3.0 who got into very reputable MPH programs, but if you have the time, money, and desire then taking some courses would be a good way to boost your chances and show your dedication. I would say (in my opinion) that the certificate program or grad level courses would be the best way to go. If you take a few more undergrad courses and do well they will be diluted into your undergrad GPA (yes, the school will be able to see that your recent coursework exhibited higher grades, but when they glance at your overall GPA it won't be as apparent since, with 130+ hours a few courses mathematically won't change your average too much). However, if you do it in a separate program (like the certificate or grad classes) then you will have a readily apparent and more recent cumulative average of your recent work. It's just my opinion, but I think if I saw an overall GPA of a 3.05 vs. an undergrad GPA of 2.9 plus a grad class GPA of say 3.85, I would think the second scenario would look better (I just made those averages up since I don't know what your exact GPA is or the number of courses you could feasibly take before applying).
Good luck!