What sort of molecules could easily cross the non-polar region occupied by the tails of the two layers of a phospholipid?
Because phospholipid tails are hydrophobic, molecules entering the region occupied by the tails must also be non-polar. The charge of a polar molecule would be repelled by the lack of charges of the phospholipid tails, so it would be very hard for a polar molecule to cross the phospholipid membrane on its own.
Examples of molecules that could cross the membrane would be oxygen gas, hydrogen gas, fluorine gas, diatomic bromine, diatomic iodide, and chlorine gas. Lipids would also be able to enter the phospholipid tail region, since they are mostly made up of nonpolar C-H bonds.