There are not many Spa and Massage locations in the Phra-Khanong district of Bangkok. The Crystal Spa is well known in the area and offers two types of massages. The first being a Swedish Massage and the second a Thai massage. The Swedish massage comes highly recommened by customers. The pricing is very reasonable and it opens at 10 in the morning until 10 at night.
You can give them a call if you wish to book (+66 2 382 4455) and they are on Exit 3 of the BTS station in Phra Khanong. The spa and massage has been around for years and well known in the area. If you have booked in at the Jasmine Resort Hotel then its not a far walk from the hotel.