planner question re index vs seqscan

I have 2 tables with indices as follows:
Table "activity" (~4000 rows) id           serial start_time   timestamp not null stop_time    timestamp not null ...
CREATE INDEX activity_start_time ON activity (start_time)CREATE INDEX activity_stop_time ON activity (stop_time)
Table "activity_hr_need" (~2000 rows) id            serial activity_id   integer not null hr_type_id    integer not
nullhr_count      integer not null ...
 
CREATE UNIQUE INDEX activity_hr_need_pkey     ON activity_hr_need (activity_id, hr_type_id)CREATE INDEX
activity_hr_need_hrtid    ON activity_hr_need (hr_type_id)CREATE INDEX activity_hr_need_aid     ON activity_hr_need
(activity_idint4_ops)
 

QUESTION:  Why doesn't the planner, just after 'vacuum analyze', use the
provided indices for this query?  How can I tweak it to use the indices?

sdb=# EXPLAIN SELECT ahrn.hr_type_id AS "Resource Type", 
sdb-#        SUM(ahrn.hr_count) AS "Planned Consulting Days"
sdb-# FROM activity a, activity_hr_need ahrn
sdb-# WHERE a.start_time::date >= '1-Jun-2000'::date
sdb-#   AND a.stop_time::date <= '1-Jul-2000'::date
sdb-#   AND ahrn.activity_id = a.id
sdb-# GROUP BY "Resource Type";
NOTICE:  QUERY PLAN:

Aggregate  (cost=243.74..244.58 rows=17 width=16) ->  Group  (cost=243.74..244.16 rows=169 width=16)       ->  Sort
(cost=243.74..243.74rows=169 width=16)             ->  Hash Join  (cost=142.65..237.50 rows=169 width=16)
   ->  Seq Scan on activity_hr_need ahrn 
 
(cost=0.00..53.58 rows=2358 width=12)                   ->  Hash  (cost=141.60..141.60 rows=420 width=4)
        ->  Seq Scan on activity a  (cost=0.00..141.60
 
rows=420 width=4)


Regards,
Ed Loehr