Projects(ProjectId, ProjectName, CustomerId, CustomerName,
Activities(Activity1Id, Activity1Name, …, ActivityNId, ActivityNName), 
Employees(Employee1Id, Employee1Name, …, EmployeeNId, EmployeeNName)
, WorkDate, TimeSpent)

Projects(ProjectId*, ProjectName, CustomerId@)
Customer(CustomerId*, CustomerName)
Activities(ActivityId, ActivityName)
Employees(EmployeeId, EmployeeName)

Solution 1 - activities are not related to employee work.
Projects_Activities(ProjectId*@, ActivityId*@)
Projects_Employees(ProjectId*@, EmployeeId*@, WorkDate, TimeSpent)
Solution 2 = Employee work on an activity.
Projects_Employees(ProjectId*@, EmployeeId*@, ActivityId@, WorkDate, TimeSpent)