I'll tell you what my ds loves- hope this helps .....
Catching newts in the pond at the park. Take a bucket or large container to put them in. Just need a kids net.
Park and picnic.
National trust type place or woods. Collect nature things like leaves, lady birds, ants, berries etc talk about what each thing is and what they do.-- birds eat ants, ants live together and they live sugar etc
Swimming
Beach for the day. Take a disposable BBQ and some buckets and a football
Blow up balloons and play with them in or outside
My ds likes to help dh do the gardening and clean the car and sort out the garage 
Take photos and collect things from whatever you do and make a collage. We stick four pieces of bright coloured a4 paper together and do a collage of the beach etc. stick couple of photos on some shells, draw some fish, put tickets on from a train ride etc.
We stick these on ds bedroom wall and he still talks about things we did two years ago Because he sees the collages to remind him.
Bbq's and burning small fires of sticks and cardboard always goes down well. Fires- especially in the evening after a busy day.
A petting zoo or farm?
Crazy golf.
Always take a juice bottle where ever you go and if you're not taking a picnic then take a couple of snacks. A sit down half way through what you're doing And a drink, snack and chat always goes down well.
But I think most of all, whatever you do, you just have to be patient and enthusiastic. Suggest climbing a tree. Race to the sea edge and jump over the little waves. A bit of slap stick like falling over the waves always goes down well too.
Have fun. Three is a lively age. Our ds1 is five now and having fun with them just gets better Nd better.