How do you stay mentally sharp when you're not able to fly? If you cancel a flight due to weather, do you just move on to other activities? Or do you take the opportunity to do some ground work?
There are a large number of resources that offer free online videos, training, and webinars. Keep pushing yourself to review old learning and add some new ones.
For example, BoldMethod has been holding some outstanding live YouTube sessions covering everything from mental math, approaches, to STARS and SIDS. And, you don't have to catch the sessions live. The sessions remain archived on their YouTube channel (see here). The great advantage of the live sessions is that through the chat function you are able to ask questions as they go along with their presentation.
There are also a myriad of webinars, videos, quizzes, and training offered through the FAA WINGS program, AOPA, and EAA:
See EAA's website here for there current, upcoming, and archive of webinars.
AOPA training page here
FAA Wings home page
Remember that completing a phase of the FAA WINGS counts as a Flight Review and is a great way to maintain a currency and proficiency program for yourself.
Lastly, don't forget the tried and true chair flying at home or in the aircraft at the hangar. When is the last time you went through the emergency checklist memory items in the cockpit and backed them up with the check list? Take the time to review the performance charts and operation of the avionics.
Maintain your aviation mental sharpness. If it's been a while since you reviewed material, start today with something as simple as reviewing a checklist and refresh your understanding of the purpose of each step. Stay proficient to continue to enjoy these views: