Here are some other hotel options that might still be available.
For the following hotels, we recommend the RET bus line 40 (between Rotterdam Centraal and TU Aula, 35 min ride)
With the same RET bus line 40, the following hotel is closer (between Blijdorpplein and TU Aula, 25 min ride)
The RET bus line 55 is direct for the following B&B (between Raadhuisplein and TU Aula, 15 min ride)
and the following hotels (between Zoetermeer Station and TU Aula, 25 min ride)
In The Hague:
For the following hotel, take a train from The Hague Central Station (Den Haag Centraal) to Delft Station, then a bus (see the list here) to the conference center.
For the following hotels, take a train from Rijswijk Station to Delft Station, then a bus (see the list here) to the conference center.