The asteroid belt
Followed by the moons and potentially rings of gas giants.
The asteroid belt is already planned in real life to be colonized alongside Mars. It’s reasonable to assume that companies and governments would start there. And it’s not like the asteroid belt is unprofitable compared to Mars, it’s probably the single most profitable place in the solar system that will be within reach very soon. Not only that bugbut asteroids can contain ice, which is the one of the most important resources you will need if you want to colonize anywhere. Ice can become fuel, breathable air, and it is just plain water, a very crucial substance for human survival.
After some time missions will focus on the moons of Jupiter and Saturn, possibly also their rings. It’s really only the next reasonable step. Venus is far too difficult to even get unmanned probes on it for more than a few hours, much less humans and I don’t expect Mercury will be any better. Mercury is very close to the sun making it also not too good of a second choice. Jupiter and Saturn are not only potential resources themselves, but their moons are not only the subject of numerous research endeavors and also have large amounts of resources. The rings of these gas giants could also provide beneficial, they are sort of like mini asteroid belts.