I would just focus on the things you have going on now. Set goals for yourself and projects with particular dates in mind and accomplish them and celebrate your accomplishments. Maybe you want to achieve a certain thing at work? Or your partner wants to finish his education? Plan a trip away to celebrate those things when you've done them so you have something to look forward to. Choose something new you want to learn or do that you haven't done before and wouldn't be able to easily do if you had a baby. Like training for a 5K or learning how to do a new skill, like cake decorating, or take a class in something you've always wanted to learn. Just make yourself busy with things you'd like to do and accomplish before you become parents. Also, schedule special days and weekends away as a couple and plan them and have that to look forward to. Doing all that stuff is really hard or near impossible once you have a child.
I haven't done something for myself really in almost 3.5 years. The most exciting thing I do is I get to go out for dinner (alone!) when I'm away traveling for work. Otherwise, everything is focused on our daughter and we don't have time or money really to enjoy things ourselves. We just had our first night alone together since she was born 3.5 years ago this past weekend. Having children is wonderful. But it means an end to a certain period in your life and you never get those days back. So just make the most of it and enjoy it all to the point that you're sick of having fun and being selfish and you've achieved all the goals you hoped to achieve before you become parents. I didn't have my daughter until I was 31, so it's a bit different. But when I was in my 20s, I was going out with friends, traveling, having weekends away to new places, eating in fun restaurants, joined a running club and ran a half marathon, bought a bike and did a few cycling events and joined a cycling group, took a job overseas for a bit, etc. All of that would be really hard now that we have a daughter and I'm glad we did it then. It meant I didn't feel like I was missing out then because I was having a lot of fun, and I don't feel like I'm missing out now because I feel like I did it all.
Also, get married and enjoy your wedding. I got pregnant about 9 months after we got married and I'm so, so glad that I wasn't (a) pregnant or (b) had a small baby at our wedding. It was a day that was totally about us and it was so fun and we had such a great time on our honeymoon. Being a parent is all-consuming. You literally don't even get to eat a meal in peace to start. We had friends who had their daughter around the time we did and then got married when she was about 18 months old. It was lovely to have her there for the wedding, but they didn't really get to spend much time alone or enjoy their guests because they had to take care of her. My friend missed like a good hour of the reception to go upstairs and feed her. You just don't get to be selfish once you have kids, so do it now and enjoy it.