As an instructor, how to show my appreciation for my TAs' hard work?
For TAs located physically close, the well-established way of showing appreciation around here is by inviting them to a joint informal dinner with good food and plenty of (alcoholic and non-alcoholic) beverages, usually with a joint pub crawl afterwards. This would typically happen at the end of the semester, optimally directly after finals, and provide a nice finishing mark on a semester of hard work and (maybe) some fun and camaraderie. These events would typically last to the early morning, leading to extremely empty offices the day after.

I am lucky to work in an university which happily picks up the bill for such events, but in previous places we have had all instructors throw money into a hat to finance this. It is not a celebration if the TAs have to pay for themselves.

Note that the celebrations are: (1) entirely voluntary (nobody needs to come, nobody needs to stay), and (2) as informal as possible.

Edit: that being said, I think the more interesting part about your question is how to show appreciation to remote TAs, and I have no good answer for this.