Zipaquirá is a city of some 70,000 people about an hour north of Bogotá, in Cundinamarca. It is Colombia's salt mining capital, and most famous for its Salt Cathedral, constructed almost entirely of salt within a now defunct mine.
Buses depart from Bogotá's Portal del Norte (numbered busses starting with the letter "B" on the Transmilenio line) for "Zipa" about every five to ten minutes, costing 4,800 pesos (roughly $2), making this a very easy day trip. The bus ride normally takes just over one hour. On the way back, just walk down to Carretera 15 from the historical center, and buses will be leaving for Bogotá during daylight hours about every 5 to 10 minutes depending on the hour and day of the week. You can also walk to the transport terminal from the main plaza and take the bus as it is leaving the terminal, by doing this you are sure to grab a seat, since a lot of people from Zipaquira work in Bogota, taking the bus in other parts of town means that seats can be ocuppied and you'll have to travel standing up. By Transmilenio from Bogotá's Portal del Norte to La Candeleria, take numbered busses starting with the letter "J."
It is also possible to take a bus from the main bus terminal instead of Portal del Norte. The trip will take about 50 minutes more. The company serving this line is named "Transportes Alianza" and their office can be located in module 3 in the main terminal. The price is the same as taking the bus from Portal del Norte. The buses leave much less often though. You can go back to the main terminal by taking a bus marked with "Terminal" in Zipaquirá.
If arriving by bus, get off at Calle 4, which will take you up a pedestrian-only street past tons of shops and cafes to the main plaza in the historical center. After admiring the cathedral and overall very charming locale, turn left on Carratera 7 until you reach the end, then turn up the hill to see the Archaeological Museum. Through the large arch next to the museum is the path up the hill to the Parque de Sal, where you will find the Salt Cathedral entrance.
You should be able to get lunch of soup, salad, beans and chicken/pork with a drink of juice for 10,000 pesos after you exit the Salt Cathedral and walk back into town.