Most funeral directors will have information about the permanent memorial choices available at local crematoria. Usually the crematorium will write to you with this information soon after the ceremony.
Many crematoria display a book of remembrance in a chapel dedicated to this purpose.
Memorial plaques can sometimes be purchased to be placed in special areas of the crematorium's gardens and sometimes seats or benches placed with a plaque dedicating them to someone's memory.
Crematoria with large grounds may offer the opportunity to sponsor the planting of a tree or other plants as a memorial. Each crematorium will have its own fees for these services and some items are time limited, e.g. a rose bush may be purchased for a period of 5 or 10 years.