Off the top of (my) head...

If you say something off the top of your head, you say it without thinking too hard about it. You mention examples that come to mind immediately… usually when you are giving someone an answer or explaining something.

1. - Do you know any phrasal verbs with get?
    - There are many… but off the top of my head, here are some 
      of the most useful: get along with, get around, get by,
       get away with, get across.

2.  - Can you recommend any really good TV series for me to 
       practice my English?
    - Of course… off the top of my head you should definitely watch: 
      Game of Thrones, Breaking Bad, The Mentalist, 
      Doctor House, Dexter and Haven.

3.  - Just off the top of your head, can you name any Hollywood actors 
       originally from Australia?
     - Mel Gibson, Heath Ledger, Hugh Jackman, Nicole Kidman, 
       Russell Crowe, Cate Blanchett, Eric Bana...