Great experience - good mix of history, experiencing the tunnels first hand and shooting war era guns
Can't go to Vietnam and not go to the jungle where the war was fought. Cu Chi tunnels are where the local guerrillas sent team USA packing unless your claustrophobic, then don't go into the tunnels!
The ingenuity of the tunnels is amazing and they are well worth exploring. The last stretch of the last tunnel which is exactly as it was in the war, not widened, hot, humid and not lit, will push your limits and you'll be relieved to see light and land after going through 50 metres.
Go shoot some guns from the war too, may be your only chance to shoot an unaltered AK-47 and RPK bigger, more powerful, on a bipod. RPK on bipod is accurate and easy to find your mark and stick to it.
AK-47 is a beast. Kicks, sight wasn't accurate and had to manually adjust, but definitely worth the experience.