It is sometimes appropriate to surmount a lantern roof with a form of decoration known as a 'finial'. Shown below are a choice of finial designs, as you can see they come in a variety of shapes and sizes. Depending on you particular property some are more appropiate to use than others. The choice of finial is very much down to personal taste, but as a guideline we would suggest the following applications:

Victorian property - a spire, obelisk
Georgian or classical property - a ball, acorn, pineapple
For a more contemporary approach it may be more appropriate to have no ornamentation at all.

See what a Town and Country Roof Lantern looks like with different ornamentation treatments by running the cursor over the list below: