Just a Personal Opinion:
"Maybe I should have gotten Sync"
Let's see $3200 more to get the Sync, then an additional $2340 to get the Navigation= $5540 total for both.
l. This is a driver's car, not a home theatre or gaming room.
2. Gee, what Modifications can I do to my car, or to my home or office for that $5540 I saved, not buying unnecessary options when I ordered my new car.
You already made the right decision, don't ever second guess it.
Why not buy some of the toys others mention above. Don't like em, throw them away the next day and buy more. It will take a while to use up that $5540 on small electronic toys...ones you can carry around with you, things you can use while that car stays parked 90 % of its life.
Oh shucks, I forgot about that rear view camera, another few hundred bucks saved. Guess maybe I'll put all that almost $6000 towards 6 months or more of car payments !