Before you have a baby, you should plan to do several things:
1. Travel. There are so many places in the world to go and you should take advantage of that now. Once you have a baby, you won't want to leave them for an 8 hour work day, much less a week in a different country. It's also much easier to travel without a child.
2. Enjoy your evenings out. Stay out as late as you can, drink as much as you want (responsibly). Once you have a baby, those nights are much less frequent and not as carefree. Enjoy your time without worrying about what time you have to be back.
3. Enjoy your friends. If your friend is truly a friend, they'll make time for you and you'll make time for them. Once you have children, there will be times when you'll have to cancel on them last minute but they'll understand.
More than anything, enjoy your freedom. When the day comes that you have responsibility in a baby, your freedom to do what you want, when you want flies right out of the window and you sometimes forget to enjoy the small moments of freedom. Don't. Enjoy the freedom now!