i personally think 2 days in Hue a bit short if you want to visit those caves

and 3 days in Hoi An a bit long
you can take a bus to sapa which is cheaper than train ride

you can book a 1/2 day tour from your hotel at around 10usd++ or you can wander around the mountain yourself. if you really cheapo you can stalk those tour group as they leave the hotel to visit those village. $120 sgd is enough for you to book a 2d1n tour
if you solo, it is better to just to go for a 1 day trip to halong bay ... feel a bit weird to be alone in the cruise
take bus 17 to old quarter and walk to the backpacker area

if you sux in navigating go take those mini van instead