Let me have a go at coming up with a dozen:

• Get rid of Harry's wand (as mentioned)
• Remove Harry's glasses (which could have been a transfigured anything, and Voldemort had just taught Harry how to dispel transfiguration by mere physical contact)
• Bind or paralyse Harry, with rope or a Death Eater spell
• Have a Death Eater Imperius Harry with a command to obey Voldemort and do nothing else
• Have a Death Eater use a Confundus Charm on Harry to make him trust Voldemort and not look for ways to escape
• Blind Harry - he doesn't need his eyes to tell Voldemort his secrets, just his ears and tongue
• On the same principle, Voldemort could happily dismember him, as long as magic was used to prevent death from blood loss or shock, and distraction from pain
• Drain Harry of magic by forcing him to cast innocuous spells
• Use illusions to disguise the number and location of the Death Eaters so that Harry is unable to come up with targeted countermeasures against them (and so are any unexpected rescuers)
• Cast a spell on Hermione as a dead man's switch - something that will not permanently hurt/kill her unless Harry does something to incapacitate Voldemort and prevent him from dispelling it in time (not violating Voldemort's promise, since he does not expect Harry to do this)
• Have one of Voldemort's innumerable horcruxes and a tied-up victim on hand so that he can come back immediately if killed (if there's a mandatory time delay, Harry doesn't know it, so this is at least a powerful bluff)
• Bring in some hostages, and kill one each time Harry starts doing anything that sounds like playing for time rather than being maximally cooperative

That took me about 15-20 minutes.