Polyurethane comes to mind. There are also products that are used to restore rotted wood like Keypoxy Wood Restorer but I've never tried them. Linseed oil also comes to mind.

In the 70's I had a path of wood (pine)stepping stones, they were very slippery, just from the dew, or wicking up moisture from the soil, so be careful.. They all rotten within about 4 years. Even pressure treated decks/stairs are slippery when wet. I now have alot of granite sink cut-outs (free from granite countertop suppliers) that I flip over to use the non-polished side (the polished side is very slippery-don't ask me how I know-ouch, ouch).

I use wood chips or crushed stone a/k/a gravel (the more irregular in shape the stone pieces are, the better) for paths.

I did the same thing with oak except mine were about six inches thick. They lasted quite a number of years and when they did rot they were removed and concrete poured in the holes that remained. pine would not last as long.