never collect anything.

my character cares not for such trivialities, not when there is evil in the world left to fight.

Precisely this. There's really not much point in collecting anything unless it's unique or extremely rare. By far the most motivating collection was Daedric armor in Morrowind because it was hand placed all over Vvardenfell and there was only one set.

In Morrowind, candles/lanterns.

In Oblivion, I collect cheese, silverware, and alcohol. The way I figure it, nobody else is going to impose prohibition in Cyrodiil, so I have to. By taking the silverware, the population has nothing to drink out of, and by taking the cheese, they have nothing to go with their wine.

Calipers.

...

If you get it, have a fishy stick.

What a drag that Cyrodilic brandy is in a potion bottle though... What is up with that?

Laziness.

Maybe. Well in Morrowind that brandy was one of my main collection items. After the infamous vintage brandy of course.

Mainly books, but I save pretty much anything that looks nice in my home(s). I usually save Varla and Welkynd stones to decorate my bookshelves with in Oblivion for instance. Depends on the character too of course, if my character is supposed to enjoy Alchemy, I decorate my house with apparatus and ingredients, while scouts and explorers bring home items that represent their travels. But books, documents, artifacts of historical value etc is a constant.

I did a lot of collecting in Morrowind as the items you found were a lot more...unique.

In specific I enjoyed collecting lights; candles, lanterns, torchs... anything that emitted light.

I liked to place them in my house and create a certain atmosphere.

I see I am not the only one who collects calipers.

Mead in Oblivion, because it's my favourite alcohol irl too. Though most bars give you a weird look when you ask if they have it in stock. I have no idea why the stuff went out of popular use, it's gorgeous.

A good friend of mine used to steal every pillow he saw in Morrowind, and ended up making a pillow fort out of them in the wilderness. He managed to work out exactly how many he could use as 'bricks' before the bottomless sack appeared and stored them all.