It's hard for you to earn bitcoin without touching alt coin and not holding bitcoin for a long time. The only way for you to earn bitcoin without touching altcoin is a long term investment or long term holding but you've said you're not into long term investment. I think the best option for you is to join a lending service where in you're going to lend someone on it and after 2 days (it's up to you or it's up the site) you're going to recived what you lend with a interest. But if you don't want lending since it's risky you can do a service like
-Signature Campaign
-Bounties
-Social Media Campaign
-Services (Photoshop, Manager etc)
This is what mostly people do there ways to make money here who involved bitcoin by joining in any kinds of campaigns and services.,