There are several stuff you can show to your friends when they come to Debrecen, but do not expect anything breathtaking.
Yellow Church: Or the Great Reformed church, which is the symbol of the City. The first single-aisle church was made around 1291 and 1311, and it was dedicated to St Andrew. After being burned down a couple of times, it got it's present shape, according to designs of Mihaly Pechy and Joszef Thaler.
It's very interesting Organ dates back to 1938, and was created by Jakob Deutchmann, a Viennese craftsman. At 1927 electric blowing action was added. This is the Church in which Kossuth Lajos announced the declaration of independence of Hungary.
The Reformed College: It dates back to 1538. It was founded by professors that used to teach in Krakow and Wittenberg. It retained it's autonomy for 250 yrs, while the building which houses it today (opposite Plaza) was built at 1803.
It's library is unique and some of the volumes housed there are extremely rare, and is the biggest Reformed Library in Hungary. It's motto "Orlando et Laborando" means Work and Pray.
Deri Museum: Not as big as the European Museums of course. The building is much more interesting than the museum itself. It was built b/w 1926 and 1928. The four statues in front of it -made by Ferenc Medgyessy- received the Grand Prix of archaeology of Paris in 1937. The one statue symbolizes Archaeology, the other Sciense, the third Ethnology while the last one is the sculpture of the artist. Three more of his statues are in the Square: Hungarian Buddha, The Venus of Debrecen and the Ballerina.
For a while still, the museum is going to host three large paintings of the world famous Munkacsy who's work can be also seen at Sz. Anna Church.
Csonka Templom (Small White Church): Built at 1600, with some additions of 1727 ( a brass globe). The Hungarian name Csonka means truncated since the bastion like top of the tower, was initially meant to be a dome that kept of falling of due to winds.
Szent Anna Cathedral: A beautiful Catholic Cathedral, that keeps on reminding you that you are actually found on Central Europe. A fire at 1811 damaged the Church which took it's final look in the 1830's. Famous artists like Munkacsy, Karoly Lotz, and Mor Than. The statue of St Anna is actually believed to represent Holy Mary.
Greek Catholic church: When the Turks advanced in the Balkans a lot of Orthodox Christian Communities in order to save themselves, adopted the Pope as their leader but kept the right to keep their Orthodox Masses. They were called Greek Catholics. Hajdu Bihar due to it's proximity to Romania is the only place in Hungary where Greek Catholics have an Episcopal seat. A very nice church nearby Szent Anna can also be seen.
KLTE (or simply Kossuth): It was erected in 1932 -it looks old though- in a neo Baroque style. The main hall, the Court of Honour has the names of the most prominent professors.
Botanical Garden: Nearby the Anatomy founded in 1928, a huge collection of Cactuses of 10.000 plants with more than 2.000 species. All in all 5.000 species can be seen.
Csokonai Theater: This building was completed at 1865. It is found on Kossuth ut. In front of it the statues of two Muses can be seen Terpsichorae and Melpomenae.
Timarhaz: Found at Nagy Gal Ut 6, a visual workshop of the trades of Debrecen, being a good inside look of the folk tradition in the town.
Cemetery // Temeto: I know it sounds grim, but according to my opinion it is the only place where you can see a typical example of Erdelyi// Carpathian architecture. The building is made entirely of wood and stone and it is decorated by it's complex, rough, almost barbaric shapes. It takes out an aggression that you feel only by looking at it. Even though Debrecen is actually a part of the Carpathian Basin, there are no examples of this architectural style typical for the Szekelyis and the Wallachians, on the other side of the borders.