Bled is a lovely spot but can be full of Brit excursion bus tourists pottering about the lakefront with their strollers... October should be fine, though, as the summer season is just about over. Bohinj is a possible alternative.
There are some fabulous cave systems around Slovenia that you can visit but possibly too far away and time-consuming for a birthday weekend.
There are some fabulous cave systems around Slovenia that you can visit but possibly too far away and time-consuming for a birthday weekend.