In the following article I try to give some overview over possible hikes and activities in and around the national park Kalkalpen in the South of Upper Austria. It was established in 1997 and lies within the Northern Limestone Alps, it contains Central Europe´s largest forested area (mainly the Reichraminger Hintergebirge). In addition it is Austria's only UNESCO world's natural heritage: 41 beech forested areas within 12 countries form the natural heritage and one part of it is the Kalkalpen national park. (The Duerrenstein Wilderness area just a few kilometres eastwards is the second half of the Austrian contribution). In 2019 the oldest beech tree in the Alps was found here, a more than 540 year old tree.
To give you a bit of an overview the following map: the national park is hardly having any mountain lodges with accommodation in the mountains but just some biwak boxes and camping spots so for sleeping if you do not want to choose these options it is best to stay in one of the neighbouring towns. Windischgarsten offers here probably the biggest variety (apartments, hotel, cheaper rooms at the protestant community center), as well for activities because you have quite some national park infrastructure there can also orientate yourself towards the Haller Mauern in the South with Grosser Pyhrgas.
Way more quiet will be stays in Molln, Rosenau or the Ennstal region.The latter is also famous for rock climbing and has the biggest density of climbing gardens in Austria. All activities are also doable as day trips from Linz or somewhere around as it is only around an hour driving.
Pretty unique and in my opinion a great offer from the national park are the two biwak/camping spots (check the green tents on the map: left one is the Steyrsteg and the right is the Weisswasser): both are equipped with toilets (filled with sawdust) and offer wood for bonfires but no further luxuries. Per person you pay 4 euros into a box, the space is limited, you are not allowed to stay longer than 2 nights and the only water comes from a river. Especially if you want to start camping in the nature, have some adventure or save money it is super to start there! From the parking lot it is just a bit more than an hour walking and there are several hiking possibilities starting from the spots.
Source: Carto.at und der Datengrundlage vom Bundesamt für Eich- und Vermessungswesen Wien ("Copyright BEV 2014, T2014/111437") - Nationalpark Kalkalpen, CC BY-SA 3.0 at, https://commons.wikimedia.org/w/index.php?curid=37610226
Access public transportation
Windischgarsten: good train connection from Linz, from the station 15min walking towards the center.
Wasserklotz: easy hike in summer. Great, rather safe snow shoe/ski touring trip in winter with awesome views, even towards the Gesaeuse mountains. Hike starts from the parking lot at the Hengstpass, it is only 600 vertical metres, so should take around 4h, similar in winter but depending as always on conditions of yourself and the path.
Hoher Nock: proper long day tour to the highest peak of the national park (1963m) which can be either done from the South/Windischgarsten or the North/Molln. There are hardly water sources so especially in summer when coming from the South take care of having enough water.
Feichtauer Seen/Blumaueralm: parking lot Bodinggraben in Molln, till the Blumaueralm it is around 100 vertical metres around a dirt road so all relaxed to walk even if you are not that fit, the ascending further to the the lakes, same path as to Hoher Nock, and there are also lodges, where you could sleep over (Feichtauhuette: self supporter and just open in summer).
Gr. Groesstenberg: perfect to combine with a night at the biwak spot Steyrsteg or just as a day trip from parking lot Haslersgatter.
Traempl/Alpstein: awesome 5h roundtrip from parking lot Bodinggraben again. Starting towards Ebenforstalm, then first towards to Alpstein (also possible to skip) and the via a relaxed meadow ridge to Traempl and from there via the Schaumbergalm back to the parking lot.
Grosser Pyhrgas (2244m): day hike from Spital, so short trip is necessary. Quite some vertical metres to keep in mind but especially the hike from Bosruckhütte is rather easy. You can also go from the Gowilalm but this is more tricky and more often with ropes secured. All huts around (Gowilalm, Hofalmhütte (1305m), Rohrauer Haus (1308m), Bosruckhütte (1036 m) are worth a visit themselves, so for a more relaxed day skipping the peak and enjoying the huts is a more than decent alternative.
Vogelgesang-Klamm: beautiful gorge in Spital, perfect for families and also to combine with Pyhrgas but then a ever more extensive day.