You could spend a lifetime in BKK and still not see it all... However, most folks that come to visit can see the highlights in 3 very full days:
1) Train to Chatuchak / Taxi back to Pak Khlong Talad (flower market area) / Royal Palace(?) / Wat Pho / River Taxi to Wat Arun / dinner on Chaopraya (one of the chain hotels on the water)
2) Drive up to Ayutthaya temples / Elephants / Lunch / River Cruise back / Muay Thai
3)Walk the malls for shopping (MBK->Paragon->etc) / street food lunch / VIP movie / massage / roof top dinner somewhere (the restaurants here are amazing)