The smallest ethnographic region of Lithuania Sudovia (Lithuanian: Sūduva, Suvalkija) is far from the least important. In the 19th century, a favorable political climate (Sudovia being part of the semi-autonomous Polish Kingdom rather than directly ruled by the Russian Empire) meant that it was in the Marijampolė high school where many luminaries of Lithuanian national revival studied. This also led to the fact that Lithuanian language is standardized based on the Sudovian dialect.
An alternative name for Sudovia is Suvalkija, literally, Land of Suvalkai (Suwalki). It is a reminiscent of that era over 100 years ago when the area was part of the Suvalkai (Suwalki) governorate of the Russian Empire.
The Sudovian plains have quite a few interesting places to offer. This is the only Lithuanian region to lack a national park and while once-rich towns have more large historical brick buildings on average than elsewhere few areas are fully authentic.
Marijampolė (pop. 44 000) is regarded to be the regional capital. It has some nice buildings including two churches, a monastery and the famous 19th century Rygiškių Jonas high school which is still among the better schools of Lithuania. However, Marijampolė old town is not intact with many newer Soviet buildings surrounding the older ones.
Other larger Sudovian towns are Kybartai, Vilkaviškis, Šakiai, Kalvarija, Kazlų Rūda.
The Southern Sudovia is in Poland today but still has a decisively enthusiastic Lithuanian spirit, characterized by places such as the Prussian-Yotvingian settlement. Never more than 20 km beyond the Lithuanian border yet over 300 km from the main Polish cities these areas are best visited from Lithuania (there is no customs control).
Sudovians are stereotypically extreme skinflints; English jokes about Scots may be translated into Lithuanian by changing "Scot" to "Sudovian".
Th original Sudovians were a Baltic tribe more different from Lithuanians. However, the area was largely abandoned in the late Medieval era as it stood on the frontline of Lithuania and Teutonic Order. After the peace settled in Sudovia was repopulated by Lithuanians from elsewhere.
Southern Sudovia is part of Poland politically but an extension of Lithuania culturally/historically. This area changed hands many times but remained in Poland after the 1920 war. Locals, however, put a great emphasis on their Lithuanian ethnicity.
A good example of that is a Prussian-Yotvingian settlement near Punsk - an atmospheric reimagination of how a prehistoric pagan Baltic village may have looked like. Constructed entirely by a single Lithuanian enthusiast over many years it has a single over-arching style which makes it even more believable (even if perhaps less authentic) than reconstructions by historians.
The "settlement" includes a small castle, watchtowers, totems, meeting circles, sacred fires, runic inscriptions, a bridge, an archery range, political-philosophical inscriptions on behalf of the extinct Prussian and Yotvingian tribes to which the owner self-identifies. Everything is built of wood while decorations are of stone and bones. The forest is both around the settlement and inside it; nearly no modern buildings are visible from anywhere. The area is no museum, however; everything may be touched and there are well-integrated benches, WCs. Lithuanian neo-pagans even hold their celebrations there.
Punsk village (pop. ~1200, 80% Lithuanians) also has two Lithuanians-related museums and one skansen. A multitude of Lithuanian symbols on the area's walls and fences are another proof of the local feelings.
Sejny (Lithuanian: Seinai) town 23 km south of Punsk has a castle-like 19th-century former Lithuanian priest seminary, while Wigry (Lithuanian: Vygriai) hosts a massive 17th-18th centuries monastery.
The main sights of Poland (Warsaw, Cracow, Torun, Gdansk) are all far away (300-800 km) and the local roads are no highways, making it impractical to combine a visit there with a Lithuanian voyage unless you plan a larger Baltic trip. The Poland's closest prime touristic place to Lithuania is the former Adolf Hitler's bunker some 170 km away.