Mid May, is usually reasonably warm (between 15 ^oC and 20 ^oC) and sunny, but watch the weather forecast before departure to Poland. Due to weather change it is impossible to say anything for sure. You can expect both tropical warm, and arctic cold with snow fall.
A raincoat may be useful, especially for the field trip.

Currency is Polish Zloty's - the exchange rate is about 4 PLN to 1 euro.
It can be exchanged in hotels and a number of bank - offices downtown.

There are many restaurants near the conference venue and Sopot city center
Lunch may cost anything between 10 and 50 PLN, dinner 50 to 150 PLN.