xiaomi fitness band。Probably not the best,

but it is cheap (around 20dollars including postage) and small enough that you can wear everyday.You do need a smartphone to use it。

其實就是高級點的計步器，跳繩是算步子的，所以我覺得玩蹦蹦床應該也能檢測到信號。我沒有蹦蹦床，這個是猜測。不過就是平時戴，也算劃算了。