I had UHaul install a trailer hitch, and rent their motorcycle trailer, which is around 4' wide and 7' long, to haul cargo around which is too cumbersome for the hatch. I have hauled 1000 lbs of sheet steel on a pallet, and just today hauled 13 boxes of wooden roses.
The trick is to get the lightest trailer which will carry your load, because the UHaul limit on the Sonic is 2000 lbs of hauled load, which includes the trailer itself. Get a 2000 lb trailer and you can haul not even a toothpick on it. But get a 1000 lb trailer and you've got 1000 lbs worth of cargo.
It's also important to follow their load distribution rules. The load should ideally be around 60% ahead of the trailer axle and 40% behind. If you have too much load well forward you're pushing down on the rear of the Sonic, and the drive and steering both come from the front end, so reducing pressure up front is bad. But worse is putting more of the weight behind the trailer axle, then the trailer gets really whippy as you try to drive around at any speed. That's what I did today, and even though my load was very light it was readily apparent that I needed to pull over and rebalance the load.
The trailer is a great deal, at $14 per day. Earlier I rented a Uhaul Pickup, which said $19.95 per day. This is a lie, the thing cost me over $60 for about a 10 mile round trip and about 6 hours between pickup and drop off. The trailer, though, costs exactly what they say it does, no mileage fee, no added costs, no need to return it full of fuel etc.
You can also take it on interstates, I went on the PA turnpike and had no troubles at speeds as high as 70, but it clearly states you should keep your speed to 55 or less. That's probably a good idea, I was just experimenting which may have been ill advised. If I'd tried it with my original backwards load, I probably would have flipped the whole thing, it was ugly handling even at 25.
The warranty is another story. The Sonic is rated, in Europe, as (if I recall correctly) being able to tow 1000 lbs, but in the US it is rated as having zero tow capacity. I suspect this means if something goes wrong while you are towing that GM will have a decent chance of claiming they are not at fault. But as a general rule, warranties don't go void as a whole, rather the specific subsections affected directly by your modification are voided, leaving the rest in place. If you are towing and your steering wheel falls into your lap, that's still their bad. But if your rear axle comes loose because you tied it to an anvil and tried to... well you see what I'm saying.
