I don't know offhand

What would offhand be in Greek in this particular usage? I suppose it means 'without thinking about it' or 'without previous thought'. Examples are:-
-I don't know offhand.
-"I can't think of a better answer offhand."
Synonyms are: without preparation, on the spur of the moment, without consideration, without checking, extempore, impromptu, ad lib.

Δε μου 'ρχεται κάποια καλύτερη απάντηση.
Αυτό μου 'ρχεται (αυθόρμητα/χωρίς δεύτερη σκέψη).
(not really the same kind of smooth attitude as "offhand"... my point being, I guess, I can't think of a true equivalent of "offhand", offhand... On the other hand, "δε μου 'ρχεται" carries a bit more of that "offhand" connotation, as opposed to, for example, "δεν μπορώ να σκεφτώ")

Additionally, maybe .. for offhand? (m_a_a_ .. good post, by the way)

στα πεταχτά;? (I've added an extra question mark)
με την πρώτη;
στα πρόχειρα;

Nice one. There may be a few things you can say that wouldn't sound wrong. Formal translations could be:

εκ του προχείρου
εκ των ενόντων

Informally:
στα πρόχειρα
χωρίς να το σκεφτώ

But I too would probably use a simpler expression, such as αυτή τη στιγμή.
Δεν ξέρω να σου πω αυτή τη στιγμή.
Δεν μπορώ να σκεφτώ κάτι άλλο αυτή τη στιγμή.

Thanks, all! I resisted the temptation to write δεν το ξέρω πρόχειρα....At least I hoped this wouldn't be too trivial a question.:)