My two favorites are President Park on Soi 24 (love the area too) and The Mayfair Executive Apartments (by Marriott,) on Lang Suang. The President Park can go for less than USD70 and has a huge rambling pool as well as gym.
They don't even ask for ID from the girls, however, I think it is a good idea to deposit it with he front desk anyway.
There is an extensive list of
guest (girl) friendly hotels in Bangkok at <-----this site.