Before I begin I should note that I may not act in the same way
if similar situations would occur. Assuming that all murders are ought
to be done by the same serial killer, he seems to be rather vibrant and
impulsive and yet able to be patient when needed.

Scenario 1.

I'm very sceptical about this task because I surely am thinking too much about various possible mistakes, like a footprint or another accidentally left clue. Since our guy seems to be a sexual killer as well, traces of semen could complicate things as well. However, I'll try to proceed with it anyway.

I silently approach the campsite and wait for the girl to temporarily walk away from it to vacate her bowels. Then I sneak behind the man, who is peacefully sitting on a folding camping chair and hit his head with a glass bottle I found near him. He is wounded and loses his consciousness. Disturbed by the noise the female returns to the camp just to be caught from behind her back and have a gun pointed at her temple. I make it clear that she is obliged to obey my orders and make her drag her friend's body to the streaming mountain river. The body is being washed away and possibly still alive man drowns to death. Then we pack their bags and turn to their car (unlike me, they weren't hiking). I force the woman to drive a few miles. After that I chloroform her and put into the trunk. I haven't done it before to reduce the unnecessary footprints that could indicate struggle near the crime scene. Then I change the license plates and drive a relatively long distance to my house. I end up home late at night when all the neighbours are asleep and nobody notices my arrival. I leave the car in my garage and take the girl to a special place of my basement for further use. In a few months I disassemble the car and dump the parts in random trash bins here and there. I also burn the shoes and the clothes I wore that day. After I accidentally kill the woman, I cut her into pieces, blend and use as a fertilizer for my vegetable garden. Maybe even sample some parts myself.

When few days pass after that weekend colleagues notice their absence and police starts to search for them. After a while they find a body on a riverbank and locate a possible place where they were camping. Since there are no things and car left and all the  footprints were washed by the rain, detectives decide that the couple had an argument and a possibly drunk girlfriend accidentally killed the man, got rid of the body, fled the scene and now is hiding. She is, but against her will.

Scenario 2

The thing that bothers me most in this scenario are the cameras, located in
various corners of streets. Let's assume I'm familiar with the streets
and know how to drive around.

I drive into the lot supposedly wishing to park there and turn the car's engine off. Then I twist the key few times stopping before the car gets started to cause an impression that the battery is drained. Then I get out of the car looking nervous and 'accidentally' notice the babe. I ask her for help and when she opens her trunk to take the cables, I chloroform her and stuff into the trunk. Then I park my car nearby and use her's to drive a long distance home. Another day I return to the parking lot by taking multiple taxi's and walking some distances so it would be harder to trace from where I came and that there wouldn't be a direct route from my home to the potential crime scene. Then I take my car and drive somewhere, for example to a supermarket or a cinema before going home. You know what happens next if you read the first share.

Police blame a local gang for her disappearance and nobody suspects a random guy living away from the scene (which probably wasn't even located) who spends some of his leisure time gardening.

I think that these scenarios are far from perfect murders and these were
my first ideas regarding the matter, so if you have anything to add,
change or suggest, please do. I won't proceed with the other cases today, maybe later.