The ACF and PACF look like this:
I think these properties suggest a specification of ARIMA(1,0,0) or ARIMA(1,0,1) although I think the former is likely more suitable. I divide the data into train and test sets by the ratio of 75/25 and estimate ARIMA(1.0.0) on the train data. However, the forecast results (on the train data, over the period of the test data) seem odd:
The forecast values have some variations for about the first week, but after that it's all homogeneous (values and confidence intervals).
What may be the cause of this problem? Is the ARIMA model likely improperly specified, e.g., a missing time trend or drift component? And will incorporating other independent variables be the next consideration?