Q: What did Voldemort do during Harry's sixth year? We have no mention of Voldemort in Harry's sixth year. So, I was wondering- what could Voldemort have been doing?

A: Although the general answer to this is "marshalling his dark forces and preparing to take over the wizarding world", there are, as you've mentioned, precious few direct mentions of Lord Voldemort in HP and the Half-Blood Prince.
Let's have a look at what we can say that he's definitely been up to, as opposed to stuff that could be the work of his minions:


*

*Practicing his occlumency

It appears that he is now employing Occlumency against you [said Dumbledore]



*

*Being "at large" (i.e not getting caught)



'Now, as everybody in this Hall knows, Lord Voldemort and his
  followers are once more at large and gaining in strength.'



*

*Being angry

'The Dark Lord is very angry,' repeated Snape quietly.



*

*Meeting with Snape

'But what use have you been?' sneered Bellatrix. 'What useful
  information have we had from you?'   'My information has been conveyed
  directly to the Dark Lord,' said Snape. 'If he chooses not to share it
  with you –'



*

*Recruiting werewolves

Fenrir Greyback is, perhaps, the most savage werewolf alive today. He
  regards it as his mission in life to bite and to contaminate as many
  people as possible; he wants to create enough werewolves to overcome
  the wizards. Voldemort has promised him prey in return for his
  services.



*

*Ordering Draco to kill Dumbledore

'And, should it prove necessary … if it seems Draco will fail …'
  whispered Narcissa (Snape's hand twitched within hers, but he did not
  draw away), 'will you carry out the deed that the Dark Lord has
  ordered Draco to perform?'

