Here's a sure-fire method involving motorcycles, which can be bunched together in a much smaller space than cars and will blow up easier as a result.
1. There's a motorcycle dealership in northern Alderny, you can find it on this map and set yourself a waypoint in the game: http://www.gta4.net/map/?lat=33.75&lng=-52.646484375&...
(edit: Updated the link.)
2. Call Packie and have him drop a bomb off for you nearby.
3. Place the bomb on a car, and drive it into the motorcycle shop.
4. There should be maybe eight motorcycles parked in this shop, use your car to pack them tightly together, preferably in a line.
5. Once you have them smushed together, park your car on one side of the line.
6. Jack another car, park it on the opposite side. This should bring you up to ten. Count and make sure you have 10 vehicles. In the comments, con2king suggests packing in a few extra cars, which couldn't hurt.
7. You might not even need it, I didn't, but stand outside of the dealership with your rocket launcher and then call Packie to detonate the bomb.
8. Everything should blow up real nice. If not, make use of your rockets quickly.