I had the opportunity recently to experience this in some form when I showed a co-worker around London recently. It was much like I had foreseen it to be, in terms of his general astonishment, but I thought his observations, those things he did take in and those he didn't comment on, were worth sharing:
(1) Heathrow Airport, larger than the entire city of Monrovia.
(2) The difficulty of finding rice at meals. Liberians are famous for their preference for rice, and I know many don't even consider a dish without rice to really even be food, and I saw this in practice where my colleague was left dissatisfied with couscous and basmati rice. He said he would recommend that Liberians travel with their own rice when going abroad, although the practicalities of having this served at a restaurant would still be an issue.
(3) We had a bit of time for sightseeing, and he wanted to see "that huge church," so I took him to St. Paul's Cathedral. It being the summer, it was quite crowded with tourists; there was a line out the door to get tickets (St. Paul charges for entry). Luckily but also a bit curiously, my colleague had little interest in seeing the inside, but mostly wanted his picture taken outside the building. So we were there about 5 minutes.
(4) The London Underground mesmerized him. "A whole city under the city," and "There are people living underneath us while we walk on top of them," were just a couple of a the many comments he made. He seemed less impressed with actually riding a train and the whole automatic entry system etc than with the general idea of inhabiting the subterranean. He commented on this repeatedly; the whole idea seemed to almost terrify him. I think he found it unnatural that people spend time under the surface of the earth like that.
(5) Back at ground level, he observed that most buildings were made "out of stone," which I took to mean the marble and portland stone façades, as opposed to the painted cement of most multi-story buildings in Liberia.
(6) Also, he didn't recognize what was sticking out of the roofs of most buildings; I hadn't anticipated this: I explained what a chimney was, which confused him a bit as he didn't see any fires lit inside any of the buildings he had been in. I realized how odd it sounds to explain that everyone has central heating but occasionally uses a fireplace, for recreational or ceremonial purposes...? How strange is that, when you think about it that way.