Two things make this game really easy.
1.) Pinpoint driving accuracy
2.) Easy pitching/chipping/flopping
Because of these two things, I wouldn't get too caught up in making a track difficult, because it's very hard to achieve this without making the course unfair. Instead, try to offer the golfer choices and make one choice slight more risky or rewarding than the other.
To add challenge I would make bigger (not freakish) greens. You are more likely to cause Par and above by making the golfer land on green and 2 or 3 putt than you are to have small greens and make the golfer chip/flop and putt. In a situation where the golfer lands off green on approach shot, they are very likely to flop or chip close enough for par every time. In most cases where the pin is tucked or otherwise close to fringe/rough the ability to chip in for birdie or eagle is high. Keep pins away from the edge and only tuck one or two or three for difficulty.
Watch your green firmness. Every pin needs to be accessible. If a golfer can't reach the pin unless it's a perfect 1 in 100 shot, its not clever or challenging, its unfair. Try to keep the area around the pin relatively flatter, give plenty of movement away from the pin. Leave yourself 4 definite spots for pins before sculpting your green. Make your par 5s reachable in 2 and accept that your Par 5s will be the easiest on the course.
I think that's all I got.
