I checked out where to park my car for WineConf and here is what I gathered.
Parking in the city centre is pretty expensive, seems to be about 40€ per day. The cheaper alternative is to use the Park and Ride parkings on the periphery.
https://www.denhaag.nl/en/parking/parking-in-the-city/park-and-ride-the-hagu...
* P+R Hoornwijck + tram 15 https://www.denhaag.nl/en/parking/parking-in-the-city/pr-and-tram-fast-and-c... https://www.openstreetmap.org/#map=17/52.04951/4.34821
This is probably the simplest solution. It's a multi-storey parking, there's one tram every 10 minutes and the trip takes about 13 minutes.
The cost is 5€ for the first day, including the tram ticket, then 7.5€ per additional day and the return tram ticket on the last day.
* P+R Station Ypenburg + NS Sprinter https://www.openstreetmap.org/#map=16/52.0553/4.3881
The parking is free but not covered and there is video surveillance. You can get downtown in 8 minutes, there is a train every 15 minutes and the ticket costs 2.30€
* P+R De Uithof + tram 4 https://www.openstreetmap.org/#map=17/52.03945/4.24470
The parking is free and in a pretty lush forest-ish environment. You can get downtown in 26 minutes, there is a tram every 10 minutes and the ticket costs 2.21€.