Unfortunately, Ryanair's booking facility is currently offline while they 'revamp' the site. (i.e. while they change it so that it will show prices which include taxes and charges). So the obvious starting point isn't available.
Additionally, Mothers' Day is next weekend, so a lot of short breaks will fully booked.
Brighton is fun at any time of the year. (Great bars, wonderful restaurants and plenty of entertainment). The rail fare, from central London, is �26 return. There are plenty of fairly cheap, but good, hotels around Kemp Town, so you're looking at a total of about �100 per person for travel & accommodation.
Paris is also wonderful all year round. Lastminute.com have deals from �127.37 per person.
Madrid is always popular. Fly out from Gatwick at 1410. (Arrive 1730). Stay at the Hotel Las Provincias for 2 nights. Return at 1810 (arrives 1925) or 2050 (arrives 2205). �118.40 per person with Lastminute.com
Start here:
http://www.lastminute.com/site/travel/holidays /citybreaks/weekend-break-browser.html?intcmp= lnav_citybreaks_weekendbrowser
Chris