The phrases "at cafe" and "in cafe" can have slightly different meanings depending on the context: "At cafe" typically implies being in the general vicinity or vicinity of a café, without necessarily being inside it. For example, if someone says they are "at the cafe," it means they are in the area near the café, such as sitting at an outdoor table or standing outside. "In cafe" specifically refers to being inside the café itself. If someone says they are "in the cafe," it means they are physically inside the establishment. Similarly, the phrases "at shop" and "in shop" can have different connotations: "At shop" generally suggests being at or near the location of a shop but not necessarily inside it. It implies being in the vicinity of the shop without specifying whether you are inside or outside. "In shop" specifically indicates being inside the shop itself. If someone says they are "in the shop," it means they are physically present within the store. Regarding the phrases "at the museum" and "in the museum": "At the museum" signifies being in the general area of the museum or in its surroundings. It implies being near the museum's location, but not necessarily inside the building. "In the museum" denotes being physically inside the museum building itself. If someone says they are "in the museum," it means they are present within the premises. Finally, the phrases "at school" and "in school" have distinct meanings: "At school" indicates being in the general area of a school, such as on the school grounds or in the vicinity of the campus. It implies being present in the school's surroundings but not necessarily inside a specific building. "In school" denotes being inside the school building itself, specifically referring to being a student or staff member who is physically present within the educational institution.

Generally, they are interchangeable for places, but "at" being more general like Ahmadi said. For simplicity, you could always say "at" for these examples as it will sound more natural. 'In' is better to use if you're specifically talking about your precise location or if you're in a moving vehicle (in a train, in a bus, in a plane). "At" is most exclusively used for events...'at a concert, at the party, at the track meet." "I'm at the cafe, museum, school, shop," etc it can be inferred that you're there somewhere in the vicinity or inside engaging in whatever normal activities you would be doing in that location. If you and a friend were meeting up at the cafe, and they were looking for you, you could said "I'm IN the cafe" to provide more specific details as to your location. But, they more or less are interchangeable and won't sound weird, generally. We also say "at the doctor" if we're at the doctor's office, but to use "in" we would say "I'm in the doctor's office" otherwise that would sound very weird...this is true generally true when using the possessive. You could say "I'm at Mike's right now," but you would have to say "I'm in Mike's house" if you were to use 'in.' So there can be some nuance. With a work meeting, "in a meeting" is better, but "at a meeting" doesn't sound overly strange.