People use all kinds of toys inside aquariums, from plastic farm animals to army men. Plastic is generally safe, although the paint is more tricky to determine (although if it is safe for children to put in their mouth, it's safe for aquariums for the most part). Bricks and other stones can be used as well. However wood is usually unsuitable - wood rots quickly and only very few kinds of wood are suitable for aquariums. Most of the soft woods will not last long at all in aquariums, and as they rot their pitch will feed a fungus that is bound to envelop them. So stay away from any kind of wooden decorations, go with ceramic or plastic...there is a ton to choose from.