I would opt for an even $100. Taxi from the airport to hotel will already be $15-20, so roundtrip is $30-40, leaving you with $60-70 to spend.

I forget how late it runs, but there's the night market outside of Ben Thanh you could go check out for late night eats, beer, and random gifts. It's virtually on the way to Bui Vien, where stuff is definitely open later.

I wouldn't walk on the way back, all you'll get to see are shuttered storefronts, not very attractive. A taxi from Bui Vien back to the Ren should be in the neighborhood of $1-2.

One more word of caution: There are counterfeit taxis out there that are colored like the big companies, so make sure the exact words "Vinasun" or "MaiLinh" are on the cab, as well as their respective phone numbers (Vinasun: 38 27 27 27; MaiLinh: 38 26 26 26).

Also, Vinasun only runs two types of vehicles: A Toyota Innova (7pax minivan) or Toyota Limo (5-pax compact car). Anything else is fake. MaiLinh runs the same two models plus a couple other different ones.

Finally, always make note of the cab number and/or driver's ID.