Tony, try not to use proper wood for the base, this can warp and split...Use MDF, plywood or chip board. Whatever you use seal it with dilluted PVA (Wood glue).
You will get lots of different answers, but I use these....Sand, Budgie grit, crushed up cereals, anything that looks like sand!lol
My method:- When the sealer is dry, re coat where you want the sand to be - all over or just here and there - sprinkle your 'sand' on and then turn the base over and tap it. The stuff will stay on where the PVA is...Job done
If you want some small rocks, go in the garden and get some the right scale and stick these on, but remember don't just plonk them on the base, it looks unnatural, rather like an iceberg, some will be below the surface.
That's basically how I do it... I hope this is of some help.
Cheers,
Ron