I have been doing MMA for about 7 weeks and have generally only been using a rear naked choke for a submission and a triangle leg lock. What are other easy to do submissions that are effective and relatively low risk?
There are lots of other chokes applicable to MMA, but for now I'd focus your attentions on the two you've been taught. They are high-percentage techniques that you should be proficient in.
If you'd like to gain understanding of other possible techniques, keep in mind that you shouldn't try to learn techniques from books or over the internet. Just watch some MMA matches. If you want to understand submissions in MMA, I recommend watching Big Nog's fights from PRIDE, or early fights from Jake Shields or Royce Gracie, or any MMA matches from Roger Gracie, Rickson Gracie, Jacare, or Demian Maia. For the triangle choke in particular, Anderson Silva versus Chael Sonnen 1 is a great demonstration.
But to answer your question directly: there's the arm triangle and guillotine, but those are easier for a new student to do poorly than a triangle or rear naked choke. I'd focus on the two you already know.
Learn submission grappling from an instructor, not from the internet. You say you've been training for 7 weeks. If you are training at a school, just keep going, you will learn more and more chokes. Maybe with the time you have free to practice at home, you can go to the gym extra and practice there? Text and video is not enough when it comes to grappling; you need the proper feeling as well. When trying some move out from the internet, you will always have questions about some little detail, how to get things right. Without an instructor or someone more experienced to show you, you will not learn anything properly and will end up crappling instead of grappling.
Basically, what I am saying is, you need an instructor to teach you how.