We could do a new member welcome. These are usually easier to do in person, often in the form of a cocktail hour or some such. For ours, we could do one online via webinar (with or without video but audio required) where new members could introduce themselves and tell everyone attending a little about themselves.
A Q&A panel webinar with some of our VM experts would also be a great idea.
One or more VM blogging webinars could recap/present the recent articles of our more active bloggers.
Our more knowledgeable members could present on different facets of VM research.
It would be great to have experts in botany, medieval studies, manuscript studies (including provenance), linguistics, cryptography, astrology, etc. presenting their sciences as related to VM research. Someone could talk/present about the radiocarbon dating and the ink tests that were performed. For the newer members at least though likely many members would find such talks interesting.
At the convention I worked for, we had hall fan tables for organizations that were less expensive than booths in the dealers and distributors rooms where items were available for purchase or marketed to industry professionals, respectively. And we also had bands performing music on hall stages in the host hotel open areas. And we had an art show and art show room where fan art was displayed, put up for auction, eligible for art show awards and sold directly. For VM Day, we could organize similar activities by having scheduled music performances presented live via webinar and threads where members can promote and include links for selling their VM-themed artwork. If we have enough art submitted, we might even be able to do an art auction, the proceeds of which could benefit the artist or their favorite charity.
We could have a create your own VM-style manuscript page.
We could have a VM AI art competition. Recognition would have to be the reward unless anyone wants to donate towards e-gift card prizes.
Members working on VM projects that require funding could host funding drives or promote VM projects that need participation.
I've seen mentioned on this site at least 1, possibly 2, VM-themed theatrical productions. Maybe some enterprising members could organize a live webinar VM play or reenactment. This could be a ticketed event.
We could share VM-themed drink and food recipes, not only presenting pics of them on VM Day but post them in advance so others could duplicate them for VM Day. You know how much we all love reproducible results.
Just a few of the ideas I've had. I'm sure I will have many more and I am probably getting WAY ahead of myself, especially for a 1-day event.