Here are little descriptions of sacred locations and holy sites.
Hiisi, Proto-Finnic *hiite, meant a holy site. It's often been connected to the meaning of sacred grove (pyhä lehto), but many hiidet (plural) were burial grounds on hilltops. Due to Christianity, the word hiisi became demonized, quite literally: it gained the meaning of "demon". It also appears in runosongs as a name of the forest god.
Sacred groves were important to communities and were shared with multiple villages. There were also sacred springs, which were important sources of water. The sacred groves might have been encircled by a fence, or there was a fenced area in the core of the grove where sacrifices were thrown in. Stepping inside this sacrifical area was forbidden. Celebrations and religious feasts were held in the sacred groves.
Many sacred groves were destroyed by Christians. Sometimes, churches were built on the same spots. Any remnants of the sacred groves are still threatened due to climate change and destruction of forests. Sacred groves are the primary type of holy site mentioned from Western Finland, but as said, most of them have been effectively destroyed. The modern state of Finland has limited interest in pre-Christian holy sites, as all other values pale in comparison to maximization of profits (unless they can be used to prop up nationalistic fantasies of course). While law requires such locations to be protected, there is a severe lack of information on them and this law is then often not followed even if on accident.
Examples of sacred springs in Western Finland include Kultalähde in Kiikala, Southwestern Finland; Neulalähde in Pirkkala, Pirkanmaa; Uhrilähde in Jämijärvi, Satakunta; and Laurinlähde in Janakkala, Kanta-Häme.
Kuppikivet or cup-marked stones were considered holy. Grain was sacrificed in their cups, and water gathered in the cups was sacred. The cup marks had likely been carved onto the stones by an earlier human population, possibly for religious reasons that are unknown. Other stones and rocks were also often sacred sites.
A sacrifical stone in Hämeenlinna was moved c. 100 meters from its original spot to a more visible spot from a car store's yard in 2010. This move was controversial, though the Finnish Heritage Agency argues thart after the move, the protection and maintenance of the stone has been easier. Hämeenlinna is extremely rich in cup-marked stones and grooves (carvings in stone intended for sharpening swords).
Karsikko means a marking made in memory of a dead person, usually on a tree (but also on rocks, walls etc). A karsikko tree might have had its lowest branches cut off. This is mostly an Eastern tradition.
A sacred tree on one's yard is called pitämyspuu. Multiple rituals involve sacrificing at the tree or having meals at it. Sometimes, a standing dead pine tree was considered sacred.
Kallohonka means a pine tree on which bear skulls were hanged on after hunting. This was in order for the bear's spirit to go back to the heavens.
Tapionpöytä, "Tapio's table", is a spruce growing without a treetop. These were altars for sacrifices for the forest god. The name would imply these were for the male forest god Tapio, but Tapio originally meant the female forest goddess, Mielikki. Finns in Northern Sweden called tapionpöytä "the cabin of the Forest Mistress" (mehänemännän tupa) while in Kainuu, if the tree was a pine instead, it was called Nyryn näre, "Nyry's pine", Nyry resembling some Karelian names for Mielikki, such as Nyrkytär.
Sacrifices were offered at the tapionpöytä to bring luck in hunting, in which case the first catch might be sacrificed. However, sacrifices were also given to protect cattle so forest animals wouldn't attack them. In this context, the object of sacrifice in Kainuu was actually cheese.
A later era altar featuring a sun wheel/cross on the top. Sacrifices (or their blood) were spread on the altar in question. Despite the sun wheel symbol, they were not specifically for Sun worship. Johan Cajanus, a church officer, destroyed the ristikannat near Kuopio in the 1600s, although these were described more as being remote trees that were sacrificed to. In case of a sun cross with a stone altar, the terms ristikanta and ristinkanta exist, while in the context of sacred trees, the word tends to be ristinkanto (kanto = "stump").
Eräpyhä means a sacred location in the wilderness, far away from the usual living spaces of people.
"Hiisi's gate", a ravine in Sotkamo, Kainuu. As hiisi got demonized with Christianity, it started to be seen as a being; in Kainuu, mainly as a giant. According to a story, this Hiisi who lived on the Vuokatinvaara hill in Sotkamo had to move away when humans arrived, as he hated the sound of the church bells. Reindeer hunters adviced him to make his new home in the ravine and promised to come worship him every July. Thus, this Hiisi also has forest god-like qualities: he takes his "grain" (animals) with him to the ravine.
Hiisi's castle is located in Sotkamo as well, while he is said to have also lived on the Kivesvaara hill in Paltamo and created the Kivesjärvi lake and Manamansalo island.
The Porokirkko or "reindeer church" on the island of Ärjä in Kainuu is believed to have originally been a Sámi holy site, sieidi. After the speakers of Sámic of the area merged with Finnish-speakers, the location continued to be known and used as a holy site.
The "cross ruin of Tohninmäki" in Töysä, South Ostrobothnia. According to local legend, there used to be a wooden image of a god with iron earrings there.