You're young enough to have many flying years ahead of you. Over time, you'll be tempted to increase your skill level as a challenge. OK, here's what I'd do. (I can't really suggest what you SHOULD do because I'm not in your shoes.) Focus on a skill you can use now and indefinitely, and is not airplane dependent. If you have the time, money and interest, probably most of the suggestions above have merit.
1. Complex and high power endorsements. Opens the door if you can cadge some right seat time,etc., and is pretty easy to do. Just time and money. Expands your appreciation for what the plane can do. Punch the ticket.
2. Upset training, or whatever you call it. Aerobatic, whatever. Enough to give you more confidence in your ability to get out of a bad spot and comfort in the envelope you can fly the airplane in. Most pilots are afraid to get over a 30° bank angle. It's nice to know the plane will do more, even if you seldom ask or need it to.
3. Glider training. Take 2-4 dual lessons. If you get hooked on soaring, get the license, otherwise take a few lessons from time to time if you feel the need/urge. It's a social activity for the most part (unless you buy a self-launch) and you need to be at the club and get along with the other pilots and help out and all that stuff. Sometimes you spend more time helping others fly than flying yourself. (I bought a self-launch.)
4. Avoid vanity ratings. Take specific training, without a ticket in mind. What? Why would one do that? Here I'm going to say something that many will disagree with. Don't get ratings you won't use. Don't get a commercial if you won't fly commercial. I did not say don't master the skill level of the commercial ticket. It's an admirable goal to be able to fly the maneuvers to the skill level of a commercial pilot. Just don't worry about getting the ticket. You might think it will help insurance. Maybe it will, but so will other documented training. And if you get into a tussle with the FAA, they will hold you to the standards of your rating even if you don't use or maintain it. They'll say they won't be don't believe them. Oh, the vanity ratings count for a flight review. Sure they do. Kind of expensive flight review.
5. Tailwheel - if you fly one and will maintain skills, get the endorsement. If not, don't worry about it. Modern airplanes have good gear and a trike is as good as a tailwheel in all but the most demanding environments. Yes, I've owned and am building one but I intend to put tundra tires on and land in soybean stubble. For grass, the right trike is OK.
6. Seaplane - I got one and am glad that I did but it was a waste of money. I don't use it. If I needed to, I'd have to get a lot of training again. I think it's a nice thing to brag about and I do, but it's not very high on the money-well-spent list for most of us. You'll have to decide for you.
7. Instrument - good to take some instruction from time to time. You need to stay proficient in it. If you'll file frequently, great. If not, well, you can get in trouble. It's a great skill, but you need to keep it sharp.
8. CFI - probably don't bother. You learn a lot learning it but teaching is poor payback and even though I shouldn't I worry about liability.
9. ME - only if you'll be able to apply it regularly.
10. Mechanic - don't laugh. Of all the skills I wish I'd developed when I as young, A&P is one I'd like to have. It gets harder and harder to be confident one is getting a competent mechanic at the time and place you need him/her. This is especially true if you own a fabric or wooden airplane. But, I suppose it's not realistic for most of us.
As others have wisely observed, it's mission dependent and your mission will change as resources and objectives change. There's nothing like a grandchild in another state to get the wife interested in a faster, more comfortable airplane, but there is seldom any way you can compete with airline rates.
My bottom line? Chase skill and not paper.
