Birishiri is a good place for relaxation but you need to lose much sweat to reach there. There are Direct bus for Birisiri or you can go via Mymenshing ( Changing buses). Train is also available upto "Jaria Zhanjail" (20 mins on bus from Birisiri). If you want to travel through train, first you need to reach Mymenshing and then need to get in a train for Jaria Zhanjail. Rail schedules are available on Bangladesh Railway page.
There are some popular resorts in Birishiri like YMCA, YWCA etc. You can stay there for as low as 400 tk ($5) one night, two persons.
Rent a rickshaw for 300/400 tk whole day to visit various attractions like Ranikong, Kohinoor Chemical Lake ( A lake with blue colored water), Bijoypur Border etc.
Ask me if you have any questions! fb.com/travelexpertsbangladesh
Thanks. Have a nice & Enjoyable trip!