Yes, it is possible. No, you are not forced to book the weekend
package for the weekend. You can book one hour anytime on weekend
within 1 month from the booking. If another person wants to make a
weekend package booking on the same weekend, then the system will
prompt the person to choose another date/package/location. It's all
on a first come first serve basis. Too bad so sad... 
Availability of car. Usually if you can confirm that you want the
car 2 weeks to 1 month in advance, then you would probably have at
least 2-3 car to choose from. It is usually the case at the
stations around my place: Clementi, B. Batok, Jurong East.
Do check with them if they have any promotion (waiver of fees, free
hours of driving) before you sign up.
Hope this helps... 
