I don't recommend hot glue because it tends to fall apart once in water for a period of time. I recommend using aquarium sealant. You can buy large bottles of it online or a smaller ones in stores (my petco carries it). Use that as glue.
What I like to do is get some stones or rocks and use the AQ sealant to build a rock bridge or cave or some other rock form. I like natural looking tanks so I tend to stay away from things that don't look too natural. But this is my taste, yours might be different.
Otherwise, +1 to blue nail.
Also here is the link to the DIY decorations thread