Look at this way Jeanie, u can cruise along at 60km/h enjoying the breeze with the sunroof open.
The Mama can appreciate the scenery.
But would a 5mth old baby be able to appreciate? Bet u a cuppa that the baby will be asleep 50% of the time, wailing 20% of the time, mama trying to coax & feed the baby 20% of the time and u'll have a little peacetime doing all da GuGuGaGa thing 10% of the time.
7yrs down the road when ur niece grew up, I'll be u another cuppa da baby (grown up by then) would have an extremely hazy memory of going to Hokkaido.
P/S: Is it supposed to be a getaway-relaxed kinda trip? May I suggest to leave the baby at home at the care of another.