Je ne peux pas penser à quelque choserien d'intéressant aujourd'hui.

Would sound more French to literally say "I can think about nothing interesting"

Il pleut beaucoup et cela m'a rendu lent mon gourdi l'esprit.

engourdir = to make slow, sluggish, numb

Pourtant, je l'aime ça parce que les nuages gris me font détendrent.

You have a whole verbal group as a subject here ("it rains much") so it would be repeated by "ceci/cela/ça" (there are exceptions, this is a general rule)
"détendre" already means "to make relax"