Leiden is easily accessible by train from the UK, France, Germany, and Belgium, as well as from several other European countries, and also by air via Amsterdam Airport Schiphol (AMS). You can search for and book international train connections to Leiden Centraal on NS International: https://www.nsinternational.com/en.
The conference hotels are right next to Leiden Centraal station.
All public transport in the Netherlands supports contactless payment: tap in AND tap out with your bank card, phone, or smartwatch for trains, buses, and trams, and use the same card/device to check in and out each time. All domestic trains in the Netherlands run like a frequent commuter metro; no advance booking is needed. Platform and schedule information can be found on https://www.ns.nl/. If you need your travel history/payment statements, or if you forget to check out and need to fix it, use the link https://zelfregelen.ovpay.nl/reisoverzicht. You can also choose to buy your train tickets on https://www.ns.nl/.
From Amsterdam Airport Schiphol (AMS): Follow the signs “Trains” to Schiphol Airport station. Direct trains to Leiden Centraal run roughly every 10 minutes (normally from the platform 5-6) and take about 16 minutes. Hold your contactless payment card or device against the NS card reader to check in before going down to the platform. Tap out when exiting the gates at Leiden Centraal.
For international train tickets booked all the way to Leiden Centraal: when transferring to a domestic NS train, you do not need to check in with your contactless card or device. Instead, use the QR code from your booking to pass through the ticket gates at Leiden Centraal.
From Amsterdam: Trains from Amsterdam Zuid to Leiden Centraal run roughly every 10 minutes and take about 23 minutes.
The Lecture Hall is about a 20-minute walk from Leiden Centraal, or a couple of stops by bus to Bio Science Park-Oost or Universiteitsterrein on bus numbers 400 (bound for Katwijk), 401 (bound for Katwijk), 183 (bound for Bio Science Park), 9, 22, 23, 30, 31, 38, 43, or 250.
For the lunch break, there is a cafeteria inside the Lecture Hall building. You can pay with your bank card. There are also other university cafeterias on campus nearby; see the list here. To dine in at a restaurant, there are more options near Leiden Centraal.
In the Faculty of Science building located behind the Lecture Hall, there are many self-study spaces for people who need them during the lunch break.
Fun activities for the free afternoon:
Van Gogh Museum (tickets need to be reserved in advance)
Katwijk beach