General advice: fitness is SO important.
Start bodyweight/weight training now. You will get strong bones to reduce chances of osteoporosis later on, and your taut muscles will support your skeleton, give you excellent and youthful posture. Your strength and tone will give you energy and make you capable of doing things you want to do, from building furniture to climbing mountains. You feel capable and amazing when you develop strength. Likewise, with flow yoga, you develop balance and flexibility which are superb skills to acquire before you start to lose them.
So do bodyweight/bootcamp/weighttraining in some form and flow yoga or similar stretchy, balancy exercise - ballet, contemporary dance, Tai Chi, in some form.
Make a bucket list and start systematically doing stuff from it. Travel now, while you are fit and young, to the most adventurous places that might require a bit more fitness, plan to do the sightseeing-based trips later. Learn skills you want to get good at. Start now and develop them. Spend money on experiences you won't regret. If your favourite singer comes to town and the tickets are £££ buy one!
Sort your finances out. Pay into your pension. Get ISAs, check your NI is up to date.
Strengthen and broaden your friendship groups. Make sure you have several - some casual, some more intimate, some physically active, some more emotionally connective. Without a partner/children, these are your key network.
Never decide not to make an effort because it's 'just you.' Make your home uplifting and comfortable to return to. Let it reflect your personality. Same with clothes. Eat healthy good food. Look after your body generally with regukar dental, eye, beauty appointments.
If there's something you really REALLY want and you don't have it in life, never assume there's only one way to get it. If you'd like to experience parenting, consider doing respite foster care or becoming a mentor to a child through a local scheme.