Tachnun & V’al Hanissim
Tachnun is not recited from Mincha of Erev Chanukah until after Chanukah.
V’al Hanissim is recited in each Amidah of Chanukah. [One should not intentionally skip V’al Hanissim to recite Kedushah or Modim with the Minyan.]
If one forgot V’al Hanissim at the appropriate place in the Amidah, he may recite it before saying Hashem’s name at the end of that Brachah. After that point, he does not recite it.
V’al Hanissim is recited each time one bentches during Chanukah. If one forgot V’al Hanissim at the appropriate place in bentching, he may recite it before saying Hashem’s name at the end of that Brachah. After that point, he may recite it as a Harachaman, as per the Nusach printed in some siddurim.