I would suggest a terramundi pot - you write something you're saving for post it in and then fill it up with pound coins. Then once full you smash it and go and spend it on your original wish. You can get them at most gifty places and it's not too modern or poncey??
What are the celtic knots type things? Could be nice as she is tying the knot as it were? Then that could be something she could keep. An antique photo frame might be nice if you could find something that suits her taste. Then she could use it to put a wedding photo in.