Location- The easiest option is to go free,but that doesn't mean it will be boring. Parks, museums and galleries are great for daytime outings but for something more different why not head online and see what you have happening near you. Britain's got talent and X factor type shows offer free tickets for there auditions which always seem to be funny and entertaining so why not make a day out of it and head over to an audition that's being held near you. This might not be for all parks but I know that my local park holds a lot of events, from concerts to funfares that are free to attend. all you do pay for is the rides and food if you choose to, otherwise it can be a nice atmosphere just to go along for the festival type vibes they have to offer. They don't just have to be a family thing, grab your friends and have a look and what's going on.
Food- doesn't have to break the bank. Begin days out with a group cooking/ food prep activity where everyone chips in their labour to make sandwiches, pasties and other on-the-go foods. Friends can gather in one persons house and have fun hand-making home-made items like cupcakes, grilled sandwiches and other hand-held foods that aren't too messy. At the end of the day food will cost money but making it at home can save lots of money especially when tourist-y locations inflate prices a lot for things like drinks and crisps.
Travel- plan the journey beforehand to avoid costly barriers that will waste time and increase expenses. Sometimes it turns out cheaper and quicker to use public transport rather that using a car and paying for the fuel so evaluate your option thoroughly.
My picks- Coming from London I can only really recommend things I know of so if you have great recommendations let me know below. I recommend Camden lock and market in the heart of London. It's a cool urban environment with amazingly dressed people and inspiring sites and landmarks to stare at. The market offers alternative and unique pieces of clothing and homeware that scream out to be bought. Even though buying costs money the actual location is somewhere that you can visit and enjoy without having the need to spend a dime.
Huge parks like Regents park or Valentines park boast large lakes with boating facilities and swans to admire in the process. On a sunny day out it can turn into a picnic in the park just admiring the array of flowers and views of the surrounding architecture. For the price of one boating adventure you can upgrade the whole day out and have a budget day out without paying a fee for every individual like most tourist-y places nearby.
Check out websites like lastmin.com or equivalents that post huge savings on exciting experiences that you can purchase for cheap discounted prices. Spa days or go-karting can become a budget option through these websites and it can be a great day out for more than half the original pricings.
Hopefully these tips help solve the troubles of having fun for cheap. If you have any more tips I'd love to know because I'm constantly bored on holidays and weekends.