here's a top floor observation room with coffee shop. The circular lounge has great views of the Golden Mount, Loha Prasat, King Prajadhipok Museum and Queen's Gallery. This area is positioned as a sort of 'break' in the guided tour, which continues with coverage of royal ceremonies and concludes with a look at the architecture of the city.
The area where the Rattanakosin Exhibition Hall is located is one of those interesting areas of Bangkok that doesn't get that many tourists, even though it's relatively accessible. The museum is open every day except Monday. Admission cost is 100 Baht (2.56 USD). On the ground floor there are two coffee shops, including a small branch of Coffee Beans by Dao, as well as some souvenir shops and a library. All of the ground floor shops can be accessed without paying admission. The Rattanakosin Exhibition Hall web site is in Thai only.