The correct sentence would be: Mitä aiotte tehdä pitkinä, pimeinä talvipäivinä? OR you could use the Finnish version of the and-word (ja) in the sentence. That way the sentence would sound better in Finnish. Mitä aiotte tehdä pitkinä ja pimeinä talvipäivinä? (long and dark winterdays) Your every Finnish word had typos/conjugation errors but it still was intelligible enough to get the meaning.

The plural marker is indeed not quite correct. In words ending in "ä" (any I can think of at least) the plural marker i replaces the ä, it's not added after it unlike in e.g. words ending in "o". Therefore pitkä -> pitkissä pimeä -> pimeissä päivä -> päivissä Note that the endings require ä instead of a because there's no a/o/u in the stem of the word. Also the inessive case is not correct here, the essive should be used (don't ask me why, that's just the way it is :). Hence, as already answered, we get "Mitä aiotte tehdä pitkinä, pimeinä talvipäivinä?" "Pitkinä ja pimeinä talvipäivinä aion opiskella vieraita kieliä!"