You are able to walk after leg day so you should be able to ride the bike at the gym. It will get some fresh blood in and help the legs heal quicker if you get in the next day. Just make it the easiest cardio day of the week, you wont break any records so dont try.
If you are consistently too sore then you should reduce your leg workout in some way. You can build things back up again later, there is an optimal ammount of work that you can do to grow and then more than that is just unnnecesary.