Sorry, it is raining in Bern, but it was also raining for the whole drive back from Aargau last night.
Even so, you may enjoy walking through the covered walkways of the old town (Lauben), have a gander at the Bundesmaus, coffee or a bite down on the river at SCHWELLENMÄTTELI.
It's an inside day, so check out the Kunsthalle, Bern Historical Museum (Heltetiaplatz), or the Paul Klee Zentrum. Any one of these can take up the better part of a day