High school players can still be with their travel team during the winter, up to the pitchers/catcher report date if they can find time from all of the other events in their life.
There are no highly competitive leagues, that I know of, for players who looking for a replacement for high school ball. The majority of travel players also play for their high school for a myriad of reasons. Level of competition or overall skill level is probably not the top reason for playing for their high school. That is a different topic in itself.
I do know that GFOD (Galligher's Field of Dreams) has a winter league for HS girls. Some teams are linked to schools, some are just a mixture of girls, some are travel teams. But the league is finished before pitchers/catchers report for thier high school.
There are some players that use leagues like GFODs and also go to some of the winter college camps to keep them busy during the winter.