Consider electricity consumption data, which is an interesting for forecasting due to having various types of seasonality (day of week, month, season), temperature, weather and daylight effects, holidays, etc.
Swissgrid, the Swiss Transmisssion System Operator (TSO), publishes a quarter-hourly timeseries for indidvidual regions as well as the entire country. The files are machine readable (Excel) and are divided into years, going back to 2011. The data is updated at the end of the month, for the previous month.
See column B of the sheet named "Zeitreihen0h15"
Total energy consumed by end users in the Swiss controlblock"
01.01.2016 00:15 1525210.59
01.01.2016 00:30 1507491.133
01.01.2016 00:45 1539664.783
01.01.2016 01:00 1518853.125
01.01.2016 01:15 1539265.905
There are many other columns for regional and national consumption and production, as well as import/export, etc. Production is a little harder to forecast because it depends on market prices in Switzerland and neighboring countries (for example, if there is lots of solar and wind power in Germany, the hourly price can go quite low, even negative, and this would likely mean you'd see Switzerland importing from Germany and very little Swiss production.)
p.s. I used similar data from ENTSO-E, which is monthly-aggregated and for all countries in the EU market (including Switzerland).
For yearly aggregated data for Switzerland, going back to 1910, check out worldenergy.ch