I see various options:
- Beak divided in multiple partially patch-covered 'lip/fingers'.
- Precise prehensile tongue.
- Fingers in wings.
- Use wings to walk to free the already hand-like feet.
- A combination of the above.
Seeing as the always-mentioned crows are able to create tools even to make other tools using mainly their beaks, I don't see this as far-fetched as other answers suggest. So,
which type of beak might the birds evolve to serve as fine manipulators in place of hands?
Like this answer suggests, a softer patch of tissue halfway along their beak would certainly help the gripping, without losing the sharp and hard usefulness.
But why stop there? This tissue would give them some lips of the sort. A more complex control (for example, closing one side of the mouth while keeping the other open) of these 'lips' would help even more with the manipulation, and could lead to a physical split of each 'lip' into two (or more) half-lips. So they would end up with a partially tissued beak divided into four (two up and two down) 'lip' parts. Those would really look (and act) like sharp-clawed fingers! If you add a precise prehensile tongue, that should do quite the trick. It could even allow for a simultaneous use of the mouth for manipulating and eating (with two 'finger/lips' holding something and the other two eating).
Of course, this is quite a weird approach (but my favorite!). So let's take a look at others.
As pointed out by bowlturner in this answer, some fingers in the wings could be used for hands (after all, they ARE hands). I would go for the pterodactyl wing which gives more fingers and it also has a more similar bone structure to the bird wing:
This approach, however, leaves the hands quite useless while in flight. And carrying loads in the wings in flight doesn't sound like a very good idea.
Bird feet, of course, are already quite good for manipulating. Many current birds use them as such.
Maybe there's a little room for improvement. Pads in the upper pads of the fingers, for example, could allow them to walk while holding something, which would be useful, althoguh probably not very practical.
A more... let's say ridiculous approach would be using the wings for walking on land, leaving the feet free to act as hands. After all, wings on land are not very useful, so you've got two unused limbs while underusing two very useful others. Those fingers suggested in the previous section would come in handy when walking on trees. One problem would be takeoff and landing, but could be solved by switching to the feet just for that moment, or taking off by dropping from a height.
All of them
Lastly, you can always combine two or more of the above. Don't be afraid of doing something weird. Mother Nature has come with very weird stuff herself.