These three stories worked really well together, since they all explore what it means to be human (and who gets to be counted as human).
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4. “The Comet,” W.E.B. Du Bois (1920)” — this is a story I’ve taught before (it’s also found in Dark Matter), and per usual, the students were very engaged with it. The plot is straightforward — a comet passes by, and we meet a black man and a white woman who may be the last survivors of humanity. As they come to grips with this, racial prejudices drop away (gender roles stay firmly entrenched, if not glorified). Lots of meat for discussion about race, class, and gender, and poignant moments keep it from being just a lecture.
“They stared a moment in silence. She had not noticed before that he was a Negro. He had not thought of her as white. She was a woman of perhaps twenty-five – rarely beautiful and richly gowned, with darkly golden hair, and jewels. Yesterday, he thought with bitterness, she would scarcely have looked at him twice. He would have been dirt beneath her silken feet. She stared at him. Of all the sorts of men she had pictured as coming to her rescue she had not dreamed of one like him. Not that he was not human, but he dwelt in a world so far from hers, so infinitely far, that he seldom even entered her thought.”
This story was a good opportunity to introduce the students to some of the basics of postcolonial theory too, the way oppressors reduce others to animals or children, marking them as not fully human, in order to justify their actions.
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5. “Desertion,” Clifford D. Simak (1944) — they liked this story too, in which a human (and his dog) are offered the opportunity for a different way of life, and find that they’re not as attached to their old lives and bodies as they might have thought.
It’s also one of the earliest stories to utilize “pantropy” (human modification for space exploration), so important in that sense — you can trace a line from here to Delany’s “Aye, and Gomorrah,” for example. This story was incorporated into his novel, _City_, which I read decades ago, but remember enjoying.
“Fowler followed, testing his legs, testing the strength in that new body of his, a bit doubtful at first, amazed a moment later, then running with a sheer joyousness that was one with the red and purple sward, with the drifting smoke of the rain across the land.”
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6. “Beyond Lies the Wub” Phillip K. Dick (1952) — this one, the students just enjoyed — it’s very funny. Human explorers meet a creature that looks like a giant pig, they plan to eat it, they learn it’s sentient, they decide to eat it anyway, and then…well, you have to read it to find out what happens next. 
“You spoke of dining on me. The taste, I am told, is good. A little fatty, but tender. But how can any lasting contact be established between your people and mine if you resort to such barbaric attitudes…”