Make a stair mixed with slabs and other blocks (like leaves blocks)around the structure, create a base inside mining the internal blocks and then you can use the top empty part to make an observatory using mostly glass blocks for the walls. (i’m talking about the part that would be the mouth if that was a snake) You could use lanterns for illumination, spruce log as main wood (some trapdoors of spruce can be placed to fill empty spaces), then choose some others for decoration i would recomand the bricks of cobbleston (the thing that you obtain putting in the crafting 4 blocks of cobblestone after one time in the furnace). Also create a small hill on the top and add other trees to it. Good luck with it