Other options might be to focus on the symbols that represent the characters and their feelings/emotions/behaviors. This type of thesis is focused on the characters rather than meaning of the story.
How about this: "Hemingway's classic story "The Hills Like White Elephants" appears to be simply a realistic description, but is in fact highly symbolic"? That would allow you to make any argument you wanted about the nature of the symbolism.
If you wanted a more specific argument, how about this: "Symbols found in both dialogue and description in Hemingway's "The Hills Like White Elephants" convey the deepest meaning of the story"? That allows you to be highly specific.
I agree with an earlier answer that the place to start is with figuring out what you think the story means, and what the themes are. For me, the thing that is interesting about Hemingway is how, for all the focus on the directness and concreteness of his prose, most of the meaning of it is left unsaid. In the case of this story, it is mostly dialog, but in fact very little real communication is happening. There are many reactions one could have to these characters: Jig is being bullied, of course, but she is also fighting back in a passive aggressive way (“I don’t care about me,” she says when she gives in). There is an emptiness in their relationship; they are talking at each other, rather than to each other, and there is a kind of intractable quality to each of them. Someone described them as “decadent,” but to me these are people who realize that they have made a tremendous mistake but don’t want to admit it or face up to each other. I don’t mean the baby, but their lives.
So a thesis statement about symbolism in the story would need to account for this. I guess another way of thinking about a thesis would be to ask some questions: How does the setting of the story, or any of the descriptive details Hemingway provides, relate to this theme? Do these things contrast with this feeling of disconnectedness or reinforce it? Can these details be seen as ironic in any way? I think if you consider these questions, you can come up with a good thesis about how Hemingway uses the landscape as a way to comment on, and distance himself from, these characters.
Another way to approach a thesis looking at symbolism in "Hills Like White Elephants" is to identify the meaning behind the story and the meaning behind the symbolism used in the story in your thesis statement.
What is your interpretation of the story's meaning? What is your interpretation of the meaning behind the symbols used? Is there a unified significance that connects each item/example of symbolism?
This is often seen as a story with a single issue at its heart, one that subtly animates a philosophical and social dilemma. The decision as to whether or not Jig should abort her pregnancy is connected to a question of modern decadence and, also, to a sense of purposeless wandering. At the train station, the couple is also at a cross-roads.
"The girl calls attention to the symbolic value of the setting and indicates that in choosing to have an abortion and to continue to drift through life they are choosing emotional and spiritual desiccation" (eNotes).
A thesis acknowledging the meaning of the story then would include ideas of choice/turning-points, binary contrasts (to indulge in aimless decadence or not, etc.) or conflicting visions of the world. The symbols of the text match up well to these ideas and can be said to articulate the story's meaning almost as much as the action and dialog do.
" Ernest Hemmingway uses symbols found in dialogue and setting to reveal that making decisions requires a great deal of thought, and can sometimes not be mutual between couples. "