the only thing you can subs is the photography - you can swap it for the honeymoon photshoot in MK during park hours or in EPCOT for an additional $50 if you choose to use an outside photographer. Altho' not an actual subs, you can also choose to use the 4 hours limo service at any time, you are not restricted to using it on your wedding day. Other than that, whatever you don't use - you lose!
Another interesting thing that I found out from our planner was that we could trade out the violinist and get the guitarist...I thought they were more strict than that...so it really doesn't hurt to ask your planner....sometimes they give answers you weren't expecting.