Improved Shield Bash from the SRD, and Agile Shield Fighter from PHB2. Agile Shield Fighter lets you TWF with a shield as if it were a light weapon, so you'll only take -2/-2 for an extra attack, and Improved Shield Bash will let you keep its AC bonus. Get a +1 bashing spiked heavy shield and that extra attack will do 2d6 base damage as a +1 enhanced weapon.
If you want to get extra attacks out of the shield, though, you'll have to take Two-Weapon Fighting (and possibly Oversized TWF to lighten the penalties, or use a light shield), since Agile Shield Fighter doesn't have an Improved-and-onwards chain to match. Agile Shield Fighter lacks the big Dex requirements of the TWF chain, though.