I’m in the camp that you shouldn’t double major if you don’t even know what your second major might be, but I’ll bite anyway.
It’s difficult to answer your question because you could double major in anything, pretty much. You could double major in English and become a science writer; you could double major in statistics and become a meteorologist; you could double major in public policy and become an environmental policymaker; you could double major in economics or business and become a commodities trader. Your interests and career goals should drive your second major (if any), not the other way around.
When you say you want to work for NASA in some way - what do you mean? NASA has HR professionals, accountants, and middle managers, but I’m assuming that’s not what you mean. You probably want to work for NASA because you want to do some work with space. NASA is currently hiring researchers: geophysicists, oceanographers, astrophysicists. They also hire engineers - looks like all their current listings are for computer engineers.
But one thing that caught my eye in their ad for astronaut candidates (because they are currently hiring new astronauts!) is that NASA is partnering with commercial space flight organizations that are also developing space exploration and travel capacity. An example is SpaceX, a private company that develops rockets and spacecraft. Lockheed-Martin, an aerospace company, is the primary contractor building the Orion spacecraft for NASA. A quick search shows that they are hiring a few people with geology expertise. In 10 years the landscape may be that private corporations are doing more space research and work than the government.
There are also positions with the EPA as an environmental protection specialist, environmental engineer, or environmental scientist/researcher. The National Oceanic and Atmospheric Administration (NOAA) also hires geophysicists, oceanographers, meteorologists and other physical scientists. The U.S. Geological Survey, obviously, also hires geologists and other related scientists.
Personally, I think statistics or math is a great second major for a geology major, because a lot of the positions that geologists can take post-college will use a lot of quantitative analysis. Modern systems for monitoring geological changes are rife with computer usage, too; yes, I understand that you want go to into the field, but every field researcher needs to be able to use the software that allows them to analyze their data after they pick it up. So computer science is an excellent second major or minor to complement geology if you are interested in technical roles within the geological/physical sciences.
You may also be interested in majoring in atmospheric sciences or earth & ocean science or something of that nature - kind of similar to geology, but there are more opportunities both government and commercially. Meteorologists and atmospheric scientists do a lot of work for aerospace companies and government agencies concerned with aerospace because predicting weather is important to flight patterns, but they also work for private corporations (weather can affect commodities! which affects business) and of course private weather-prediction companies like AccuWeather or the Weather Channel.