The French-Belgian high-speed train operator Thalys is turning 25 years old and is therefore selling many tickets for only €25 each way. You can book these tickets until June 14, 2021 and travel between June 28 and September 3.
This Thalys sale is valid for routes connecting the following cities:
Some routes are even cheaper than the advertised €25 (e.g. between Brussels and Germany). All bookings can be modified for a €5 charge + a possible fare difference.
You will have free wifi during your train trip, as well as a power socket to charge your devices. These trains are also very fast, e.g. getting you from Paris to Brussels in less than 90 minutes.