PicMonkey is a great online tool I would use with my 6th graders to make online vocabulary and for students to create visual collages of different science concepts.
Thinglink is awesome as kids can easily make an interactive page to share with the class and I can make a Thinglink for a fun class review and embed this on my website and my canvas site.
Soundcloud isn't age appropriate for middle school children. There are too many items for school age kids to see that they shouldn't see on this site.
There are many assessment options out on the internet to use, however, if our school district is pushing for everyone to use Canvas, I can't see the point in learning and using a new tool that is exclusively for assessment when we need to learn how to use Canvas. There just isn't enough time to learn and use these online assessment tools when we haven't mastered Canvas.