Test Drive Unlimited

These are much more difficult than the race and speed challenges. In general, be sure to pick the best car, tune it as much as you can and try to avoid collisions which will totally stop your car. If the damage bar exists for that challenge, be careful to not hit cars or stay off the road too long (though briefly exiting the road is better than the -20 penalty for hitting a car) and remember to use the back button if you exit the road, which instantly transports you back to the road. You might have to replay some of these multiple times before you get the gold, both to get some practice and because the cars spawn randomly - one slow driver in the wrong place can wreck your run.

There is a very good guide here: http://guides.gamepressure.com/testdriveunlimited/

Most of the challenges are not very hard if you have a good car and can control it. There are a few that are a lot more difficult:

Tantalus - One of the hardest challenges in the game. Use a level 3 Enzo, Pagani or Edonis and don't try to go too fast in the beginning. It's more important to not hit cars than to go really fast. Control is key in this challenge.

The Maserati Trophy - You'll need a level 3 MC12 which is a pretty lousy car for class A. Watch out for the hard left after the third checkpoint and floor it for most of the straighter parts after that. You have very little room for error on this challenge so it's important to keep moving forward without collisions or missing turns.

Triumph Time Attack - Drive to the side of the road so you can avoid the cars and push forward to lower yourself on the bike and increase your speed when you get a straight section. Be careful of the hairpin about halfway through and the S at the end. You'll want to keep your speed up without flying off the track or hitting cars. Easily the hardest bike challenge of the game with little room for error.

The Pass of Good Hope - You'll need a good level 3 car for this. I did it with the RUF RGT but Maserati and Aston Martin also have some good class D cars you can use for this race. There's no damage but but time will be very tight so you have to avoid collisions and keep your speed up. Watch for the hard left about halfway through since it's easy to miss and the bumpy parts where it's easy to crest a hill only to crash into the oncoming traffic.

All the others should give you little trouble as long as you use a good car and tune it up. Even the Millionaire's Challenge is easy, if long. You can use the 180 mph in traffic speed challenge in the center of the island for quick cash if you need it. Just use a fast class A car, turn around at the start and go in the opposite direction. With a car like the Saleen S7 or a McLaren you can make a million dollars in a few minutes.