Won't you be...won't you be my trainer? :)

Not a trainer but there are many things you can do. YouTube has so many workout options available for free.

Yoga and planking can definitely help you build a strong core, increase flexibility and maintain mobility. It's not just a fad but it is beneficial. You'll either love it or hate it. I happen to love it. Yoga with Adriene is a great resource for beginner to advanced yoga. Also, I highly recommend investing in a yoga block. You can find Hula Hoop and Jump rope videos for free as well.

You also might want to up your walking game. Get outside if weather permits. Free videos like Leslie Sansone's walking videos are a good way to get your steps in as well as elevate your heart rate.

If adding a bit of muscle is a goal, add body weight exercises then work up to weights. You won't get bulky. You will feel stronger and better. Fitness Blender is a free internet site that allows you to search for custom workouts from beginner to advanced and you can specify what equipment you have available and even find workouts with no equipment at all. When you're ready to begin adding weights, look into nerd fitness. There is another site quite a few people here use for weight lifting but I can't think of the name of it. Hopefully, someone else will chime in on that.

If you are looking for a guided yoga/strength training program and are willing to pay for it, I highly recommend DDPY yoga. The workouts are excellent and you only need a mat and a block. It's a good place to start for someone who is completely new to working out and building strength. It is a tiered program and allows anyone at any fitness level to get started. FYI - I completely ignore the nutrition plan of DDPY because it is not based on actual science and is much too restrictive for me. Since you're not looking to lose weight anyway, that shouldn't matter for you.

A couple final notes, start following any program that includes some body weight exercise, planks and pushups and you'll be surprised how quickly you can gain strength. One to five pound weights will likely be too light very quickly (except for walking - they can add a bit to your walking workouts. Since you are not interested in losing weight, ignore the scale, instead, take your measurements and track those. You may actually gain weight if you are doing strength training. That's not a bad thing. Your measurements and picture progress will give you a much better visual of your progress than your weight.

Good luck!

It sounds like you have varied interests and goals. Instead of taking too much on at once pick one thing to incorporate into your routine over the next few weeks. If it’s yoga, look for a beginner class near you at a studio (or even possibly for free at your community center). Then go a couple times per week while you continue your walking on other days. If you’re nervous about a class (like me) try out some beginner videos on YouTube. I personally like Yoga with Adrienne but there are others out there.

For strength training without weight you can start with bodyweight exercises or use two bottled waters instead of dumbbells initially. I like HASfit I’m YouTube and my sister also recommends Fitness Blender.