tautline hitch is an old favorite of mine. I think I first learned to tie about 50 years ago when I was tying guyout lines for the canopy of our old canvas umbrella tents. It is a wonderful line for tarps and such. Easy to tie, easy to untie and holds really well even in shock cord.
The figure 8 knot is fairly easy to untie dependoing on the line. The trick is to bend it back and forth to loosen the loops. Start in a similar way to untying a bowline, where you flex a loop to loosen it up. Sometimes it helps to have a small marlin spike to work into the knot to loosen it up.