Stay at the IC and you're just around the corner from the post office and cathedral. You can tick those of your list pretty quickly. Most of the other places are (un)comfortable walking distance (and that way you get to see a bit more of Saigon), or a short taxi ride. Not the tunnels though - and I'm afraid I'ver never worked up the interest to see these, but I do know that you'll have no trouble finding agencies that can slot you into a group or set you up with a car and guide. Your travel budget is the key.

Given that you've only got a day and a half, I wonder if you might be better seeing the easy to visit places in the city, and just having a general mooch around, sampling local food and beverages.

I've no idea where HCMC's jade buddha is.