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 null
hr_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_id int4_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..243.74 rows=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