What do you enjoy doing? Focus on yourself and doing things you like to do, travel, sleep in, just relish all of that sort of stuff and enjoy it. It's incredibly hard once you have kids to do the most basic things for yourself, like working out, or reading a book or taking a class and learning something new, etc. Your time is no longer your own and unless you have free childcare, like a willing grandparent nearby, it's really hard to do anything for yourself, like going to the gym or having a morning off to sleep and not wake up at 6am or to sign up for a class on whatever when you have small kids. If you've always wanted to run a 5K or learn to ski or sail a boat or take a painting class or whatever, do it now.
Plan a trip somewhere you've wanted to go. Travel will not be the same once you have kids. My husband and I traveled and lived overseas a lot before we had our daughter, and we took one last trip backpacking around India the month before we started to TTC. It was wonderful. We couldn't do that sort of thing now. Traveling now is going away and doing family-friendly things during the day, and then being stuck in a holiday cottage or hotel with a baby monitor all evening, no more going out for drinks or a nice meal out or exploring a new city at night. It's a very different experience and you miss out on things you'd get to do if you didn't have kids. See lots of your friends too, including the ones that already have kids. It's easy to visit people in the evening when only one of you has kids (they can put theirs to bed before you come over). When you both have kids, meeting up for dinner is harder. But just generally spend lots of time with your friends because you won't have as much time for them in the future. I find it's a lot harder now to coordinate because people either need to come to us for dinner at our house (fine if they live nearby, harder if they don't) or we need to plan ridiculously far in advance to meet up so we can get a babysitter and by the time, the day rolls around, something comes up for them and we end up not seeing them. So make lots of time for friends, go on weekends away with them, go out late nights with them, etc.
Have lots of nights out with your partner and sleep in and spend time together. Your relationship changes and the time you have for each other will get smaller and less spontaneous. If we want an evening out for dinner alone, it takes about 2 months of advance planning to secure a babysitter. Our time just isn't our own anymore. So go out to all the restaurants you've want to go to, travel and spend weekends with friends, stay out all night, sleep in, do that sort of stuff until your sick of it. By the time we did start to TTC, I was pretty sick of it and it wasn't all that exciting anymore and that was really good. I'm glad we made the most of it (though what I wouldn't do for a late night out these days, especially one where I could sleep past 7am).
Just find stuff you enjoy doing and do it to death while you wait. It will make the transition to parenthood a lot easier when you don't feel like you're missing out too much because you really made the most of doing everything you could. That's really it. You don't need to do much to prepare your body, unless you smoke or have some other medical issue that needs to be sorted first. And you don't need to know much about babies to have one. I'd never really even held a baby before I had my daughter. You figure it out.
