Eulalia, I'm thinking of parties too for younger son. Here's some ideas, culled from my experiences the first time round.
I hate cooking so, for me at least the aim is to do no food preparation whatsoever during the party apart from pouring drinks. So all food is laid out in advance and covered.
No cooking also because to enjoy myself and relax I need to give my concentration 100% to the the birthday boy and the guests. Earlier postings here have recommended organising other adult helpers. Definitely agree! And to keep things as simple as possible, I would personally rule out having a barbeque. I know my carnivorous son would be extremely attracted to anything smelling of sizzling sausages. The constant shooing away form the hot bits would be too stressful for me.
Crisps: not too many at a time, if you have them. Keep topping up supplies and put them in lots of small bowls so no one child can hog the crisps.
As others have said here, pass the parcel ( made of newspaper) is just about playable with 2 year olds. Sweets as presents between the layors.
Any other games I've tried have failed. Musical statues may be OK, though. No harm in having a go. Also avoid prizes - the losers are liable to have tantrums.
I personally wouldn't open presents until later. I suppose it all depends how good your guests are at sharing. There's bound to be one favourite toy that everyone wants.
Activities you could think of doing are: A lucky dip - burying little parcels of sweets or something in the sandpit. If I remember you said on another board that you were buying one?
Also you could use the sandpit for a small car play area. My son loves small cars, I take a bag of them everywhere. Gets lots and lots so everyone can join in. Borrow from friends etc. or buy them secondhand from boot sales if you have time. Personally I think battered metal cars look much more authentic and interesting than shiny new ones.
If you can procure an old parachute, then your guests can have fun crawling under it. Lots of games of hide and seek.
Or make a den for them indoors or outdoors.
For a very cheap party theme, if you have a paddling pool, make little paper boats and your guests can play sailors. Then add a paper boat to every good bye bag at the end of the party. And still nautical, get a jelly, add some soft fish-shaped jelly sweets and angellica (seaweed) and put them in to make a jelly rockpool.
Can't think of anything more. Just lots of kitchen wipes and bin bags and loo roll...
Good luck