There is nothing unsafe about play sand. I live in North America and use Quikrete Play Sand. It is available from Home Depot and Lowe's, and possibly elsewhere. There is no safer sand; contamination is just as likely if not moreso for any "aquarium" sand or gravel. I corresponded with Quikrete a few years back about their play sand; it is processed to be safe for children, who as someone mentioned in another thread are often putting stuff in their mouths; the manufacturer said the machines are washed prior to processing play sand to avoid problems.
I personally would never use any form of glue (aside from aquarium-safe silicon which is a "glue" in its own right as it bonds things together) inside an aquarium. That doesn't mean it is not "safe," but there is no reason to use glue so don't.
Plants from outdoors are never safe. Terrestrial, obviously as Colin said. Aquatic are also very unsafe depending where you live. Those of us in temperate regions will find that native aquatic plants cannot manage under permanent warmth, they need the winter "die off" period and in the aquarium I've never heard of any temperate plants managing well. I have some of the temperate species of Frogbit which I purchased from a fish store many years ago (nine or ten now) as Amazon Frogbit (the true tropic species) but after it flowered it was obviously the temperate species. It has survived, just, but never done as well as my other floating plants, obviously because it is not a true tropical species.
The other and more dangerous aspect of collecting local plants is one of disease and pathogens. Watercourses across the earth have unique pathogens. Tropical waters have pathogens differing from those in temperate watercourses. Fish living in tropical watercourses build up immunities to the pathogens living in those watercourses--if this were not so, the fish would all have died out millions of years ago. Same holds for fish living in temperate watercourses. In both cases, the fish are not immune to the pathogens of the other watercourse. This is one of the big dangers of releasing any aquarium living creature (fish, plant, snail, shrimp, etc) into the natural ecosystem. Even if the creature does not itself decimate the local fauna, it can introduce new pathogens that may very well do so.