Trips in Hanoi and Ho Chi Minh

Can anybody recommend any day trips out of Hanoi and Ho Chi Minh and who the operators are?

Please tell us your interests.
However:
Halong Bay, Tam Coc/Ninh Binh and Perfume Pagoda from Hanoi

Mekong delta, Cuchi Tunnel and Cao dai combined, Vung Tau by hydrofoil from HCMC