Deeper inside the ship of Maima are some mobs starting around 950m and going up to ~2.3b. You'll naturally progress to them if you were fighting in this area already from the 100m. You probably still can't complete the quest here yet however.
This is one of the most popular areas for the billions. Small Yak's start right at 1b and successive mobs are: 3b, 5b, and 7.5b. Each mob drops some pelts and if you collect enough you can turn them all in for layered gear. The gear is not particularly great but can help be filler for all the stuff you'll eventually get later. This gear is meant as a lesson about how layered gear works within the game.
Temple of the Eternal Dragon
The Ginyu mobs start at 750m, these can be found UP from Solrun. You'll need to be flying and it will take you to another area. Once you work your way inside the temple you can find some more mobs. Be careful as the wraiths (1.5b) are aggro inside the first area. The Phantom's on the west side are 2.5b. The ghosts on the east side are 3.5b. You can complete a decently challenging quest in this area for a great aura. Layer 128 about.
The Polluted Delta
I don't travel through this area often. Enemies start at 1.5b and go up near 10b. Each mini zone in the area has a chance to spawn an enemy when you walk in, as well as turning you on PK red. PK isn't very punishing in this game but if you want to avoid it completely steer clear of this zone. As you progress further back through the various zones within the area the mobs will get stronger and stronger. Be careful as a few of them can pack a punch. There's a PK quest here for some races to get their forms if they needed to, although with the new skill system you can just practice them at a master.
This area starts around 1b and has mobs wandering around to about 10b. This area is okay for gaining but the mobs can be a bit annoying to find and there's not a ton of them. Inside the dome are a few more stronger mobs, but they're androids and cannot be scanned.
This is a great area to gain in once you've reached at least 5b and can fight the small husks. Wander around a bit to discover a suit that lets you breathe in space. You probably won't be able to strictly stick to this area for gaining because there are some large gaps between the different mobs. It is a great area to fill in gaps and come back to from other areas though.
This area is located on the northside of HFIL, you can IT to Cell and walk north, or go to snakeway and fall off to get here. The mobs start around 2-3b and go up a bit higher, however there is a PL restriction in this area. If you wander in at too high of a current powerlevel you will be killed instantly. It is a bit annoying to navigate but if you need a few more mobs to kill around 3b there is some near the entrance.
This area is not an area itself, but is the small village that is within the Namekian Wilds. There are two different sets of nameks, one within the houses and ones that roam the streets. The ones outside are 2.5b, and inside are 4b. They can use some powerful attacks occasionally but otherwise are not too hard to kill. This area can be used to supplement other areas but probably doesn't stand particularly well on its own.