How to Cover Giant Sewer Vent
That concrete riser really should be buried, ideally. So give the idea of dumping a lot of dirt there some serious thought.

Disguising the pipe (and the riser if you don't bury it) has many possible methods, starting at the low end with painting it, since sewer-pipe green stands out quite a bit. As is, you might make the pair resemble a broken-off stump with a branch sticking up using brown/gray/black paints.

Presumably you don't want tree roots right there, but some artificial foliage might help, or some annual vines trained on strings. Going more involved, you put a (removable) windmill or lighthouse (or other decorative motif structure) over them.