What you need is toys, or not even really toys, but items that let them explore the laws of physics, that's what they are fascinated by at this age IME. So things which can go in and out of containers (and lots of different sized containers). Balls which can roll. Things with wheels. Tubes and slopes to put things down. Stuff like stacking rings, stacking cups, shape sorters, wooden blocks, duplo. Things that open and close. Things that only go together one way but are fairly simple, like those wooden jigsaw puzzles with the handles. With more difficult toys you will have to do it with them a few times until they get the hang of it. A few basic role play toys are helpful as well or safe items they can use to replicate everyday life. Pretending to make phone calls, "fix" things with a screwdriver, stir food in a saucepan, drink coffee like Mummy - all great fun and really sweet to watch.
Also, at this age they are starting to develop preferences towards things. For example DS2 loves tractors, cars and farm animals. He has a little friend who is obsessed with dogs. So if you know what their favourite thing is, you can look specifically for books, toys, pictures etc that incorporate those things and leave them out for them.
Rotating toys is definitely a good idea. I also find for some reason just moving the location of them makes them interesting, so I swap some between the living room and his bedroom, or take some onto our balcony and suddenly they are like brand new toys.
I let mine have free rein of all board books but avoid anything pop-up or lift-the-flap, so that the books he has access to are pretty much indestructable. Don't be precious about the books getting damaged - they're supposed to be interacted with and IME they get chewed and thrown about before they start to use them to look at the pictures and then "fake read", but they will do both of these things in time if you regularly read books to them so they know how they work. They do need a bit of help to recognise which direction the spine goes and will try to close books the wrong way a lot. It's not on purpose and if you encourage them to ask for help (I did this by teaching a sign) they generally will before they break the book. If the spine does break, tape it. Admittedly this is easier for me to cope with as I get most of his books second hand paying very little and plenty of them are in battered condition already. But they really can take a LOT of rough play before they become unreadable.
It's tricky when second hand sales are essentially impossible at the moment, but you can still buy simple toys and books at the supermarket, or online, if you have the money to do so. Colouring (scribbling on things) also starts to be interesting at this age. Print out some colouring sheets with easily recognisable items on them (google "free colouring page farm") or whatever keyword you like, or buy a cheap supermarket colouring book. There are various child friendly colour options from chunky pencils/crayons to washable pens. Look online (beware cheap foreign imports which might contain harmful chemicals, stick to known brands - RoseArt, Crayola, Staedetler etc)
Snacks which consist of tiny pieces that they have to pick up one by one are long lasting and an activity at this age. Cheerios, rice crispies/other cereal, sweetcorn, sultanas, raisins, dried cranberries, pretzels broken up into pieces, freeze dried fruit like bananas/strawberries/raspberries (can often find in baby snack aisle) all work well.
They love to feel that they are "helping" or being like Mummy/Daddy at this age so adapting any household tasks so that they can "help" is brilliant in about 3 different ways - you get stuff done - they are learning that housework is a collaborative effort - you are entertaining them and keeping them out of trouble - it fills up loads of time. Plus because you've "done something" with them, you can have a guilt free coffee break after that. Good ones for them to "help" with IME are washing up of plastic/wooden, unbreakable items, wiping floors/tables/windows with a cloth, warm water and some mild soap, putting washing into the machine, taking wet washing out of the machine, hanging stuff up onto lower bars of an airer. (Yeah, they scrunch it up and leave it there, but you can fix it, also try showing them how to shake the creases out first!) Tidying up toys into a box, carrying things to another room (especially if they have a toy pram/cart/trolley to use, or lightweight box to push along the floor!) If they start to do the opposite of what you want, just gently show them and explain in simple terms "I want these clothes to stay on the line so they get dry! Can you get me another one from the basket?" - obviously they don't understand everything but they tend to be eager to please at this age, plus if you're treating it as an activity to do with them rather than a job you have to get done, you'll feel less annoyed about this and it can be really funny to see what things they think are helpful and how they mimic you.
You are allowed out for "exercise" - just following a toddler around seeing where they want to go and what they want to investigate is great fun. Ditch the buggy (it just gets in the way), bring reins or a back carrier if you feel the need to contain them at times, or carry them to a safe place to start, or drive to a safe place like a wooded area. Avoid parks - too many surfaces other people have touched, and they can't access the play equipment yet anyway, but an open field, canal/stream, not-busy beach, quiet street or woodland is great. Obviously they have no concept of social distancing but you can steer them or hold them back while people pass you.
Janet Lansbury's advice about how to help toddlers learn to play independently is also helpful, this is a good starting point to her approach :) www.janetlansbury.com/2013/05/stop-entertaining-your-toddler-in-3-steps-2/