Sell Doffs. Even some whites are in high demand and will net you around 100k per doff. Best ones that I know of are Security Officers, Assault Squad officers and Sensor Officers. There may be a few others that get you some money.
You can recruit new doffs at the academy. If you have a lot of alts, you get o many doffs that this will easily net you several million.
Downside, it's incredibly boring.
Or just get a fast ship and try to make 4 rounds of Tour the Universe. That should also get you nearly a million at least.