Part time job
Getting another job can be a good way to earn extra money. You may be able to find regular evening or weekend work that will pay you extra. If you are in the UK be sure to tell the tax office as you will otherwise be taxed at a very high rate on your second job. You will also need to check with your employer that they are happy with you working two jobs as some companies will not allow it.
There are many ways, these days that you will be able to sell things. You can use local papers, table top sales and car boot sales, but many people these days use the Internet and there are many sites such as auction sites, local social media sites and places like this where you can sell items. You may have to post things out if you decide to sell online, but you will have a larger market for your items and therefore could potentially sell a lot more things.
It could easily be possible to freelance in the area of work that you do or in something else where you have skills. Whether you do this locally and advertise through word of mouth, local papers, flyers etc or use a website where you can pick up freelance work will probably depend on the sort of thing you do and whether it is a skill that is more likely to be used locally. There are many websites that you can use to find freelance work and it is certainly worth looking at some of them and seeing whether you think that they will be useful to you.
Advertising revenue from blogs/videos etc
Some people make quite a good income form advertising revenue on various sites that they are part of. It may be from advertising on blogs, websites, videos or other online means. Some people can make a lot of money from this but it does take a lot of hard work and commitment. You will need to be committed, do lots of updates and promotion and possibly experimentation with tracking what is popular and what is not. However, with skill and perseverance it can be a good earner.
It can be really good to try to get some overtime in the job that you are currently doing. This can be easier as it is guaranteed income, unlike many of the other things mentioned. You will also know what you are doing so it will be easier and you may even be lucky enough to be paid at a higher rate than you normally are. It is always worth asking if there is any.