Welcome mum2Fergus and lizardqueenie
I have 2 DS's, also 3yo and (nearly) 1yo like Jenduck. We have luckily just got an allotment so we spend as much time as possible down there, weather permitting. Do you have a garden? Maybe you could section off a bit for DD to grow her own things/play in mud? DS1 loves his little patch of our garden and is very proud of his sunflowers, pumpkins and courgettes that he has grown.
We do lots of sticking/glueing and painting here, lots of hand and footprints for DS2. I leave their creations to dry, and once they're forgotten about I cut out the best bits and recycle them into gifttags for presents - the grandparents love this more than the actual pressy I think I stock up on craft things in The Works, Poundland, This is It, or on Amazon.
A good website is www.bbc.co.uk/cbeebies/ - we love the "I can cook" recipes :)
I also take food with me, my changing bag is usually stocked with boxes of raisins, dried apricots, mini cheddars, bottled water (and a little packet of haribo for DS1 is he is good) all food that can stay in there from day to day, and just refill as needed and stuff I can pick at too if peckish
I have also moved all my beneficial store cards - boots advantage,nectar etc - into a separate card wallet inside the change bag, so I can go out with debit cards, but still collect my points and things.
A friend of mine set up a group on Facebook "Mums of xx" 6 months ago, and it now has 500+ members, all mummies from the local area. We now have regular meet ups/outings etc and I've met some great friends. We help each other out with babysitting and things aswell - maybe something you could look into in your area or set one up?