There are a few bars and restaurants in Reus that offer work however you will need to speak Spanish. If you are looking for seasonal work and you don't speak Spanish you should head for the coast - to resorts like Salou. There is a large tourist population there throughout the summer, and the hotels, bars and restaurants all need workers who can communicate with the tourists in their own language.
In Reus centre the type of jobs available are more in the administrative and office field. There is an office called Ocaso Assegurances just off the Placa Prim on the Raval de Santa Ana. When I passed they had administrative work advertised in the window.
You can also search the official employment office webpage 'work in Spain'. You need to be able to speak Spanish, but it has jobs available all over Spain in many types of industry. 'work in Spain' website.
The wages in Spain are generally lower than in the UK. If you are working in a restaurant or bar then be prepared to earn between €800.00 - €1,100 per month plus your tips. It pays to be nice to your customers because the tips can make up a significant proportion of your wages.
Some bars and restaurants may not be willing to give you a contract if you are only working for a few months in the summer. Legally they should give you one, or you should register yourself as self-employed with the tax office and social security office. Many people do work the summer without a contract, but it's not officially correct, so be aware of your rights.
For office work in Reus town centre expect to earn up to €1,300 per month.