I do find these things help too.
Having a bit of a stash/stock of interesting but cheap activities - craft stuff, jigsaws, board games/card games, consumables like playdoh or hama beads, add ons for things we already have e.g. wooden train sets, Lego. I pick these up at charity shops, or craft shop discount area, or middle of Aldi/Lidl or whatever. Mostly, I do this without the DC around so I can keep my stash hidden and if a day is going a bit pear shaped, I have something to bring out where the novelty value is enough to intrigue them. If I'm feeling a bit too blah to actually be super enthusiastic and joining in, I can generally make myself a cup of tea, stick some music on that I like and sit with them while they do it rather than trying to be a nursery school teacher.
If I am out with the DC and we are in a charity shop/cheap shop, I try to have a little bit of money available for them to spend, just with a budget, because again novelty value tends to extend the play time a bit. But that keeps them happier to come along on errands and it then occupies them for a bit when we get back home.
Books of "rainy day ideas" or kids' experiments or similar are also vv helpful, though many tend to involve a bit of prep/set up so it can help to take a moment at a weekend or a regular point once or twice during the week to look through the book and mark an activity or two you want to do so you can do the prep, e.g. freeze small toys inside ice blocks, or buy bicarbonate of soda and food colouring for an experiment etc.
Having regular clear-out or tidy-up sessions where I go through all the drawers/shelves/cupboards where we keep toys, collate similar things together, put any stray pieces in a designated "lost pieces box", weed out broken items and things which are outgrown or no longer interesting. Sometimes I have to sneak things out, but usually just dredging up the stuff that was all mixed in the bottom of a drawer and therefore unplayable tends to make it interesting again, and they get intrigued by it. We also have a set of the Ikea Trofast drawers both upstairs and downstairs with the plastic trays/tubs, so in doing this collation exercise I can swap whether things are stored upstairs or downstairs which also helps mix things up and keep it fresh for them.
Are your DC into the small lego yet? It might be a bit tricky for them still but what I would recommend doing if/when you do get any lego is get a ring binder with four rings and a load of pockets divided into halves and quarters. (Can buy on ebay if local stationery store has nothing). These are perfect size for Lego instructions, so you can put the instructions into the folder and mix up all the pieces in a big tub.
Then go onto the website Rebrickable on a computer, tablet or your phone and set up an account and input all the sets that you have. It does some magic and collates all the pieces you have together and then you can run a search where it will show you what other Lego sets you can build with the pieces that you have. Both official sets, and creations other people have made with custom instructions. The custom ones are usually free to download although some have a small fee. It's brilliant and really helps keep interest in Lego if your kids aren't into making up their own things.
If you have any local attractions where they do an annual pass or summer pass, that is often well worth the money esp if youngest can still get in for free. Having unlimited visits is often better with young children, because it means you can pop in for 30 mins/an hour, rather than feeling like you have to "get your money's worth" and stay for a whole day which is often exhausting and requires so much prep with picnics and so on.
And yeah TV definitely has its place - we also have an old Nintendo Switch which they love to play multiplayer games on - Mario Kart or Mario Odyssey were things my two could play together when the youngest was 3. (In Mario Odyssey there is a guided mode and in the 2-player version, the second player doesn't have to do very much but feels like they are helping.)
Mario Party (the mini games) they are also getting into now youngest is that much older, and Minecraft, though this can be dicey as a 3yo will typically struggle with the controls, or get bored and start breaking blocks and the older one will get frustrated. That can work as a three player with me helping the youngest, but equally can cause explosive arguments. 9/10 Minecraft is a game that my 6yo plays on his own while the younger one does something with me or watches TV instead.