Plan your week with a mix of activities eg play groups, stay at home time, library etc, so you always have things you can do, to keep yourself from getting bored.
When thinking about treats, focus on ones that are the longest lasting eg a coffee is great, but once you've drink it, it's gone. A book, fancy hand cream, or nail varnish can be used multiple times.
i do online surveys and scan my receipts for rewards. it takes ages to get rewards, i look forward to them.
if you have a Waitrose you can get to, they do free coffees with a purchase. i'd get a bakery item and have a coffee for lunch. Asda and IKEA also have cheap coffees.
collect any shopping reward points you can and use them for treats, For example, the body shop give you a £5 voucher for your birthday, Boots recycling scheme give you £5 in points when you recycle 5 items and spend £10 (with twins, i guess that will be quite easy).
I can download books and magazines from my library for free on my tablet or phone,
Learn new skills - try new food recipes, learn to bake, knit or craft. things can be done from home and you get a great sense of satisfaction.