Harassing in the lane:
With Untouchable, a good heal, great MS and a ranged attack with a decent attack animation, Enchantress is arguably one of the best heroes in the game at trading harass. Especially if you have Orb of Venom, constantly attacking your opponents is a great idea. You should be more careful while you have lower levels of Untouchable, but the more levels you get the more powerful you will become at this.
Passive harassing is not something that only an offlane Enchantress could do – walking into a lane as a roamer (ideally with a creep) and staying there a couple of minutes to pressure the enemies is a great idea.
Another reason Enchantress is great at harassing and a powerful laner is that she can use creeps to help her in the lane. How you harass depends on what kind of creep you have:
The best example is the Harpy Stormcrafter who has a great active ability to harass with. Chain Lightning has a very low CD, high range, decent damage (140) and a very low mana cost of (given the Harpy’s large mana pool). This means it is an incredibly powerful ability to simply spam in the lane.
The big Wildwing is also useful for harassing. You can hide the creep close to the lane and use the Tornado, trying to control it and keep it on top of your opponents for as long as possible in order to deal maximum damage. Since your opponents will likely try to dodge the Tornado, you might end up zoning them out of last hit or even XP range for the duration.
The last creep that is worth mentioning for harassing is the Satyr Mindstealer, who has a useful Mana Burn ability. It is far less spammable than the Harpy’s Lightning, however, the 100 mana per cast burned is quite significant, especially on heroes with low mana pools.
Harassing with the other large creeps is usually simpler – just hit the enemy with them and try not to feed them to the tower. Letting a large creep loose on an under-farmed support could create a lot of problems him/her.