The crack was tight, I wasn't able to enter it in my ski gear for more than a foot or two, but I suspect one might be able to enter it in the summer. The crack was pretty big though, high, long, and also open from the other end.
The ice cave was a nice experience -- also free -- but felt a bit artificial. It is in fact under an (artificially?) piled area of snow and ice next to the top station. But inside the frozen blocks of ice, with flowers inside were very beautiful.
Coordinates for the crack cave: N 46.78861 E 8.41332. The ice cave is at N 46.77087 E 8.42355, at the top of Titlis, accessible from the lowest level of the lift & restaurant building.
Beautiful frozen flowers at the ice cave at the top of Titlis: