Hot water or boiling water and that's it. Ive NEVER had a problem picking up something from outside EVER. You have to be aware though, some things like pine trees are not safe, but Oak branches & leaves, Beech sticks, leaves & cones are safe. 99.9% of the rocks you find are safe, etc. Any dried out wood like driftwood shouldnt be a problem either. At least Ive never had a problem with any.
Some of my most favorite memories of keeping fish are actually "collecting adventures". My gramp used to go to the lake or stream and collect driftwood, rocks,
sand, plants, etc., for his tanks and I used to help him. Now we've been doing this with our son since he was 2ish. Kids love finding sticks and rocks and bringing them back. When my son was 4 he had a 30 long in his bedroom FULL of rocks and sticks he had collected and 50ish
Guppies. Now in his room among his 4 tanks is a 40 gallon Convict tank with a 4' tall stump that he found while we were fishing. We go on adventures all the time to collect sand and sticks and driftwood, rocks, plants, etc. Makes the hobby WAY more fun.