13. Teen aliens on a "joy ride." They've come to prank us, do donuts in the atmosphere in their ship, and generally make a really big nuisance of themselves... Maybe they do the equivalent of TP the planet...
Or as a hunting expedition... (See also
Hunter Planet: The All Australian Roleplaying Game, where the aliens, who look largely like stuffed toys, are taking safari hunts in Northern Australia...)
9.Trade. Trade useless trinkets to the hairless savages for valuable metals, minerals, radioactives, interesting biologicals, etc...
There is no point in trading for anything but biologicals - the metals, minerals, radioactives, and pretty much all other mono-elemental molecules other than Oxygen, Florine, and a couple others are obtainable more easily off world than on - the most useful super-silicates are deep beneath the crust on earth, but are plentifully found in asteroids at/near surface. If they can get to our system, they can raid the materials easier than getting them from us.
10. Soldiers. Through slavery or treaty, used as low tech cannon fodder/meat shields against another interstellar civilization.
Soldiers, voluntary or not, are essentially slaves. When it's compulsory, it's a period of slavery. When volunteer, in almost all, it's a term indenture.
11. Entertainment. Battle Arenas, Death Game Shows, Intelligent Alien Zoo Exhibit, etc...
Again, just variations on enslavement.
A few poli-sci majors I knew in the 90s wanted to ban the practice of all experimentation upon and placing in zoos of the other apes on the grounds that keeping animals that intelligent in captivity is slavery.
I'll note that having seen the interactions of several ASL-trained apes with their keepers after having taken 2nd year ASL... they were signing about as clearly as my nephews (who are children of deaf adults) did at age 4. Or me after 1 semester of ASL.
They also have been demonstrated to have a concept of self - they can spot things on themselves and groom themselves in a mirror. (Lemurs don't always pass that one. Some dogs and cats do, some don't...)