Work log attributes are used to associate additional information for work logs. There are 5 types of attribute categories which are text, boolean, duration, integer, decimal, single select. Here some recommendations for work log attributes:
Invoice ID: Id of invoice to bill a client easily from work logs.
Billable: Not every time spent is billable. Add a boolean attribute to specify which effort is billable and which effort is not billable.
Work Type: Type of work done, for example testing, review etc.
Overtime: May be you are paid for overtime, and if this is the case you can keep track of this with a simple boolean attribute.
Of course actual attributes will depend on your requirements, these are just common suggestions.