Complex Date Range Gaps

Hi,

I have a table with date ranges for activitys. I need to get the userid , firstdayidle, lastdayidle for when a user has been idle in the last 90 days.
it gets tricky since it also must include users that have no records in activity table or only one record so it has 2 idle periods, etc

I have a hard time wrapping my head around the logic, weither using a datetables or not, so any help would be appreaciated

--my first step is to get the activitys that enter the last 90 days
select * from Activity2
where (enddate > DateAdd(d, -90, getDate()) AND enddate < getDate()) OR (startdate > DateAdd(d, -90, getDate()) AND startdate < getDate())

CREATE TABLE [dbo].[InternalUser](
[userid] [int],-- IDENTITY(1,1) NOT NULL,
[fullname] [varchar](50)
}
insert into [InternalUser] values(1,'a')
insert into [InternalUser] values(2,'b')
insert into [InternalUser] values(3,'c')
insert into [InternalUser] values(4,'d');
insert into [InternalUser] values(5,'e');
insert into [InternalUser] values(6,'f');

CREATE TABLE [dbo].[Activity2](
[activityid] [int] NOT NULL, --IDENTITY(1,1)
[userid] [int] NOT NULL,
[startdate] [datetime] NULL,
[enddate] [datetime] NULL
)

insert into [Activity2] values(1,1,'2007-02-15 00:00:00.000','2008-03-15 00:00:00.000')
insert into [Activity2] values(2,2,'2007-01-01 00:00:00.000','2008-01-02 00:00:00.000')
insert into [Activity2] values(3,2,'2008-01-20 00:00:00.000','2008-04-10 00:00:00.000')
insert into [Activity2] values(4,3,'2008-02-20 00:00:00.000','2008-10-10 00:00:00.000')
insert into [Activity2] values(5,4,'2007-01-01 00:00:00.000','2008-01-16 00:00:00.000')