For seasonal storage and rare event backup: hydrogen burned in a combined cycle plant. Hydrogen can be stored underground for maybe $1/kWh of storage capacity.

High voltage dc interconnects? Importing Danish wind energy is one of the projects supposed to keep the lights on after an exit from nuclear in Belgium.

Yeah... I'm not too sure about those Danish winds blowing when the Belgian ones aren't. This last week would have been a major issue for example.

I believe that hydro for storage is pretty much maxed out already.

On-river hydro storage is maxed out, but the potential for off-river pumped hydro is vast.

http://re100.eng.anu.edu.au/global/