I was thinking language arts was easier for self-directed enrichment as long as the school doesn't get in the way. As a kid I did a lot of what I thought of as "sideways" work, personal enrichment. DW and I had talked about an unschooling approach and ways to track things for DS6 when we weren't sure we'd get him into an accelerated program. Quite a bit depends on being self-directed/motivated.
Here's some ideas:
1) Set a goal for # new words to learn each week
2) Research etymologies for words in regular class
3) If there isn't one, try to start a spelling bee team
4) Choose alternate books that are thematically similar to the one in class, do comparison/contrast in book reports
5) Draw analogies to mythology, etc.
6) Start a book club
7) Start a literary magazine
8) Learn sentence diagramming
9) Get a manual of style and learn and apply the rules
10) Get state standards for the next X grades and mark off current ability and track progress to others
11) Write
12) Write
13) Write
