Here's what I personally use:

https://smile.amazon.com/gp/product/B008FUUQJW - Extra sealing ring to use when you make sweet things to avoid transferring any smelly savoryness from the other ring.

https://smile.amazon.com/gp/product/B06XKPBT1Z/ - Steaming basket, of course.

https://smile.amazon.com/gp/product/B01KHHFCKS/ - A 7" springform pan to make Cheesecake #17 in. (If you only get one thing, this is the one. Because cheesecake.)

https://smile.amazon.com/gp/product/B008FUJ2LK/ - Tempered glass lid for slow cooking and/or letting people peer into the pot in awe while food is being kept warm.

And if you just want to buy fun things, the mini mitts are nice for getting the cheesecake out right away: https://smile.amazon.com/gp/product/B012D33BD4 And not catching on fire when you want to remove the inner pot or hold it in place while you saute.

I also bought these stackable pans the other day (https://smile.amazon.com/gp/product/B01M130JJL/) but so far all I've used them for was a dual meatloaf. I like them but I can't really think of a lot of uses for them at the moment.

I use this set: https://www.amazon.com/gp/product/B01M130JJL/ref=oh_aui_detailpage_o03_s01?ie=UTF8&psc=1
Actually I use it a lot because I like rice with my meals.

Seconding the "pot in pot" (aka PIP) method. You can do rice at the same time or other veggies as well. Obviously it is affected by the cooking times for the other dishes. You can search around online and find a lot of people who stack oven safe containers with different foods in the Instant Pot and cook a meal at once. I have done some of the recipes from Jill Selkowitz at This Old Gal and have seen others. I have [these] (https://www.amazon.com/ekovana-Stackable-Stainless-Pressure-Steamer/dp/B01M130JJL) metal tiffin pans that I use for this purpose also. You don't need to buy special pans, though. Anything that is oven safe and will fit below the 'max fill' line and allow you to put the trivet and some water in the bottom of the pot. I have cooked rice this way several times. I also freeze cooked rice and reheat from frozen with a little water in the microwave. You can reheat in the pot, too, if you don't have a microwave.

I use the Ekovana Stackable Stainless Steel Pressure Cooker Steamer Insert Pans from Amazon.
https://www.amazon.com/gp/aw/d/B01M130JJL/ref=mp_s_a_1_1?ie=UTF8&qid=1521468122&sr=8-1&pi=AC_SX236_SY340_FMwebp_QL65&keywords=ekovana&dpPl=1&dpID=51owU4b-K3L&ref=plSrch

Any thoughts on using stackable pans like this instead?

That's the model I have too.

If/when I need to cook more than one thing at the same time (PiP--pot in pot), I'll use various ways and various bowls/containers/steaming baskets/trivets/risers. It's just a matter of figuring out what you have, how much the foods need to be separated, and how everything will fit and balance. Sometimes I'll just cook one thing in the bottom of the inner pot and a second thing on the wire rack that comes with the unit (or on a container on it) or in/on a steamer basket, trivet, etc, with its own legs, or use other risers, or use my stackable bamboo steamers.

https://www.reddit.com/r/instantpot/comments/4iwsd3/which_silicone_steamer_basket_to_choose/d331tff

Some owners in the Instant Pot Community FB group like the 2-part stackable unit called Ekovana Stainless Steel Stacking Inserts. Or some Indian stores/etc carry something similar:
https://www.amazon.com/ekovana-Stackable-Stainless-Pressure-Steamer/dp/B01M130JJL
https://www.facebook.com/groups/InstantPotCommunity/search/?query=Ekovana%20

If you are trying to do multiple dishes at once, a lot of people on the IP forums recommend using something like these:

https://www.amazon.com/ekovana-Stackable-Stainless-Steamer-Insert/dp/B01M130JJL/ref=sr_1_3?ie=UTF8&qid=1509542983&sr=8-3&keywords=instant+pot+pan

Do you cook the tomato sauce in one of these:
https://www.amazon.com/ekovana-Stackable-Stainless-Steamer-Insert/dp/B01M130JJL/ref=sr_1_2?s=home-garden&ie=UTF8&qid=1505868413&sr=1-2&keywords=instant+pot+in+pot

You probably want THESE

You can also Google stainless steel stackers for pressure cookers

Stackable Stainless Steel Pressure Cooker Steamer Insert Pans

Look, it is a proven fact that pressure cooking preserves heat sensitive nutrients better than other forms due to vastly reduced cooking times.

Overall food is softer/well cooked, specially with lentils, legumes and beans (I follow a plant based diet).

I actually like that I can cook really faster and tastier with pressure cookers. Reduced gas bills too. Specially with pan racks it is so easy to do multiple things at once.

I actually use one of these types of cookers too. No fancy bells/whistles. Cheap and cheerful, with built in safety.

How nice of you to research your gift. Your choice will depend largely upon how many people she usually cooks for.

I have the 8 quart Ultra (I usually feed 9-10 people) and don't regret it at all. This is the only version with the knob and I love how easy and intuitive it is.

On Amazon, if you click on an Instant Pot and scroll down, there are charts comparing them. Like here.

I regularly use the glass lid you have to purchase separately. There are other gadgets you can get for it, like stackable pots and extra sealing rings (because yours will eventually smell bad), among other things.

You can just type in "Instant Pot Accessories" on Amazon and see what is available.