Do the things you love - spend time with family, loved ones, travel, whatever floats your boat. Because life is short and you don't know what's round the corner. I wish I'd spent more time with the people I've lost in the last decade (I'm 50) than I did.
Exercise - get into a good routine and include strength work. Your 50 plus self with thank you.
Look at your nutrition - do you have a health diet and is there enough protein and fibre in it? Do you have weight to lose - if so, start working out what sustainable changes you should make today.
Arm yourself with knowledge about peri menopause, know what symptoms to watch out for and what treatments are available. Understand the potential benefits of HRT. I wish I'd been better educated and started HRT 5 years earlier. You don't have to fear it, but take advantage of the greater awareness and understand their is now.
Look at your alcohol intake - if it's too high, start trying to cut it down. It gets harder for your body to process!
If it's your bag, look at your skincare regime and what might be worth changing as you get older.
My 40s were great btw, there's lot to look forward to - especially giving a few less fucks about some things!