Hot glue gun and glue
At this point, your mushroom could be finished if you want it to stay natural. You could also pull out some paints to decorate it. I plan to use inexpensive acrylic paints to make the mushroom/acorn top red then paint little white dots.
Once you're done with your mushroom, have fun decorating with it! Try making a mushroom "terrarium" in a little glass jar, or gather a few together with string in a tiny mushroom bouquet. I think little red mushrooms will look especially cute in a fairy garden, maybe even with a little glittery fairy dust. There are so many fun possibilities!