In my turn, can I just take an "accessible" card into my hand and not play it yet?
No, you may not keep the card for later use.
The rules say

On your turn, you must choose an “accessible” card in the card
structure and play it. An accessible card is a card which isn’t
partially covered by other cards.
You can play the card you’ve chosen in one of three different ways: •

Construct the Building • 2. Discard the card to obtain coins • 3. Construct a Wonder


There is nothing in the rules about keeping cards to play later.  You make an immediate choice from the 3 options listed.