I have a 2.5 and a 14 month old and we go to mum and toddler groups, go to the park, beech, sandpit, fill the washing up bowl and water play (in the garden), play with cotton reels, glue and stick, go swimming, singing, read books, talk, clean up together, cook together - DS2 gets the pastry and plays with it in high chair, play dough and on and on
we generally have fun and do lots of outdoor stuff if the weather is good
nothing wrong with leaving him to get on with stuff imo, so long as he's happy!
at this age,
staying in - ds loved "washing up" - still does at nearly 3, duplo - do it together and try to see if you can use up all the blocks, lots of baking - pastry and mixing, playdough though they do have a tendency to eat it at this stage, making dens, dancing about to music, a few montessori activities - very simple and fun for a bit of quit focussed "work" together.
going out - playgroups, baby signing, library, park, beach, cinema, lots of trips to art galleries and museums, lunch in cafes, swimming.
play football or cricket - I'm not joking my boy is 18m/o too and nuts about cricket. Make a den and curl up in it , he may come and see what your up to , blow bubbles - both of you. Weed the garden, boys more enthusiastic than carefull but he can pull plants up. go blackberry picking. Make a scrap book - you do the cutting up , he "choses" where on the page you glue what picture.
lego- I suspect you already do that tho looking at your name.
Mums and tots... if you live nearish one you might want to try it.
An 18m/o who is self amusing is a wonderfull thing tho, try not to lose him.