To the south of the house is the driveway (which takes up almost all of the property on that side), and to the southwest is the garage. Eventually I'd like to remove the garage and driveway in favor of a larger garden, but that is far away and may never happen.
I can see why you are pining to get rid of the southern driveway and SW garage. Your garage is in the Southwest and I'm assuming this is a source of shade on your small garden to the west of your house, and that your house is also a source of shade in the latter part of the day. Is that right? Bummer. And your driveway is on the South... double bummer. Yeah... I would definitely consider getting rid of those. Perhaps you can move your garage to the South East Corner?
I tried to do a sketch of your description (on paper for the purpose of clarity for me), but there are some missing elements. I have to assume your roadway is to the East from your description.
What is the gap between your driveway and your South property line? None at all? If there is a little gap, great! If not, is it possible to break up that edge of the driveway (sacrificing a foot) and direct water along the south line towards the East, to the roadway with a drainage ditch/pipe/drainrock/landscape fabric? See below for details.
Where are your house and garage roof's draining? This should be directed as far as you can to the East before it hits the ground, and possibly out of the property as described above.
What is the gap between your garage and your property line? If there is a space there, a drainage ditch along the western property line could take the excess hill water, away from the rest of your property to either the north indentation that you speak of or the South as I have described above. If there is not much room between on the North of your house, how do you plan to keep the increased water in the indentation from coming into your basement? Is the slope away from the house that great that you are not concerned? This might be a concern in my mind; just sayin. Also you may end up with unhappy neighbors if you make this indentation into a much wetter place.
fill the trench with gravel (what kind? will gravel work? do I need landscape cloth or a pipe or anything else?)
When you build your trench/path system, you really should use perforated drain pipe. Use round stones (called drain rock), not crushed angular gravel, and put that around and on top of the pipe, and do put landscape fabric on top of the rock to keep silt/dirt out of the system, then, put your path paving stones. This gravel/pipe/landscape fabric will improve drainage greatly. Here's a blurry but sufficient youtube video of a guy doing it. Skip the first minute and watch until minute 4ish.