For toddlers I’d say
Some blocks or bricks. These can be used to build but also interact with other toys, they can become food in a bowl, furniture for dolls, rocks for dinosaurs. You can use them in water, for counting and sorting…I think coloured ones offer more play opportunities but some people prefer plain wood.
Water play. Doesn’t have to be toys but jugs/cups/ladles that they can scoop and pour with. Something that floats and something that sinks. A sponge or similar that can soak up water and be squeezed.
Bags/baskets/boxes. Amazing how long they’ll spend filling up a bag and emptying it again. Carrying it somewhere else and starting again.
A doll. Preferably one with removable clothing. They often use dolls to help them make sense of the world and to reenact real life experiences.
Cars and trains. They learn so much from playing with these
Ball and track. For preschoolers a marble run but for younger toddlers who would try to eat the marbles you can buy hugely expensive bigger sets or just use a ball and some cardboard tubes or packaging. You can also use your blocks to make tracks or supports for the track.
Also something for mark making, paint, pens etc and offer messy play to experience different textures etc. And let him play with everyday items eg pots and pans, a hairbrush etc
You can add to it as they get older and develop interests or if you see something is missing from their play eg a threading toy or some pegs to help with fine motor skills.
As they get older if they’re really in to a character then by all means buy them some figures but have a generic dolls house for them to play with them in and some generic figures as well instead of buying whole themed sets. Peppa pig will only ever be played with as Peppa pig but a plain doll could be anyone or anything and encourages more imaginative play rather than just reenacting episodes of a tv show.
I have four children of my own and have worked in early years for many years and some children instantly play really imaginatively with open ended toys and others need a bit of prompting to realise for example that the blocks can be other things. Once shown though they do play well with them but some children do need suggestions for different uses, the blocks being used as food in the kitchen doesn’t automatically translate to them realising they could also be used as money in the shop or as a bed for a doll. It’s always been really interesting to me to see how only offering open ended toys in a setting plays out with the different children.
At home the toys that have really gone the distance include Lego, magnetic tiles, wooden train set, marble run and some cars. Most of them are a bit old for your child at the moment but worth considering as he gets older.