Travelling to Thailand 3 weeks feb/march

Me and a friend male 22 and 23 travelling to Thailand for 3 weeks from Manchester airport on 19th of feb. not as organised as we would like to be!!! We have only are flights booked. ( our rough plan) Arriving at bangkok Monday morning book in hotel throw bags in off out have a look round maybe book a trip.stay there couple days head up north possibly on night train, hoping to do jungle treks, an see elephants all that stuff etc. then head backdown to island ko samui, Phuket. Ko phi phi etc. then back to bangkok to flay home. Both have good budgets so will be kind of going with the flow when we get there. We are both chilled relaxed laidback.we know are plan will be completely different when we get there!! Just hoping for great experience! Would like to here if anyone has any similar plans or suggest any ideas cheers!