"Skin deep" and "under the skin" are a reasonably common pair. Here, "skin" is not limited to an organic sense, but can include things like the skin of an airplane or a computer's case. It may not do so well with relatively homogenous things, or where the concealing outer layer is quite thick in relation to what's inside; talking about the amazing crystals hidden within the "skin" of an unopened geode would, I feel, be odd, because rocks don't have skins: they're just rocks all the way through (and the part that looks different is pretty deep inside).
"On the surface" and either "under the surface" (if there's enough space between the uses to keep the similar words from clashing) or "deep down" also work, expressing something a bit more like an ocean, where the top appears fairly barren, but there are millions of fish and microorganisms swimming below.
In either case, it's often more useful to take just one of the pair and allow the other to be implied.
For your specific example, I might take something like the latter pair, like this:
On the surface, this programming language appears to lack such a feature, but if you look deeper, it has a much more powerful one.
Is salesman the natural term to use about a male appliance store employee who walks around trying to help customers?
"Salesman" is a perfectly fine and natural English language expression. But it is outdated in many parts of the world.
Having worked in retail, I can tell you that these terms (i.e., salesman, salesperson) are commonly used for people who work in the stores as well as for those who visit customers/clients in their homes/offices.
Merriam-Webster:
a person (especially a man) whose job is to sell a product or service in a given territory, in a store, or by telephone
Cambridge:
a person, usually a man, whose job is to sell products in a store, or by visiting or phoning customers or possible customers:
a car/computer salesman
a bond/equity/insurance salesman
But try not to refer to a female salesperson as a salesman - that may very well get you into trouble.
Note that in some regions, retail businesses prefer using the term "sales associate" or "sales assistant" as opposed to "salesman" or "salesperson". The "associate" or "assistant" variants are considered by some to be a bit more respectful, and to reflect higher status and expertise than the other terms. This is however anecdotal (based on my experience), and there could very well be regional differences.
This is from Indeed:
A Sales Associate, or Shop Assistant, promotes the company’s products and services and helps to answer questions customers may have about the company’s activities. Their duties include helping customers make buying decisions, servicing existing customers, and overseeing purchase agreements.
Sales Associates are part of the sales team. They are usually front-liners, meaning that they represent the voice of the business and are the first to interact with the customers. They help customers meet their buying needs. Sales Associates are also responsible for drafting and submitting periodical reports based on sales and other occurrences in the company.
Another similar role is that of a "sales/retail clerk". They have slightly different duties (including some overlap with those of a cashier and a teller). See Wikipedia.
Best Answer
totter and stagger are what you want, for instance:
He staggered home, drunk and intoxicated.