For a large group for a cheap dinner the best place to get really good food would probably be New Tayyabs:
http://www.tayyabs.co.uk/ - tons of Indian food, cooked the proper way, for very little money. Of course there are thousands of restaurants in London.. for those who are interested in food you can eat in a really good 2* Michelin place for like £30 a head with a set lunch; world class food etc. If you genuinely want more in depth food recommendations I can help with that side of things.
http://www.timeout.com/london/ - a good guide to finding out what's happening at specific dates×/read reviews for food.
http://www.visitlondon.com/events/index - does the same thing for happenings.
To echo gif83 - the British Museum is relentlessly awesome, as is the British Library. HMS Belfast on the river is open to visitors and is one of my favourite attractions in London. The IMAX cinema near Waterloo isn't too expensive and films can be worth seeing there. Greenwich is fun - park, observatory, maritime museum, pretty views. Denmark Street is amusing - non stop guitar shops. The best burgers in the known galaxy are to be had in Camden at Hache. The majority of the museums have free entry so for those who rarely/never visit London I would encourage you to visit at least the British and Natural History. They're world beaters. Harrods is an attraction (try playing spot-the-most-expensive-item and find that 400k table), as is Fortnum and Mason. Uhm, Borough Market is cool.
I've done the London Eye and it's cool, but probably wouldn't appeal to everyone. There's the London Zoo too, but again.. not for everyone.
I could also turn up. If anything interesting gets planned (or just to abuse wtbw). I can also provide a real Rubik's Cube as alluded to above.
Edit: TheBlasphemer, you can get a coach straight from Stansted to Victoria in central London for £10, comfortable, convenient, doesn't take too long (30-50 minutes depending on traffic). And you get the diminished hassle of using Stansted rather than Gatwick (Gatwick and Heathrow are both a pain in the rear end, I find).
Second edit: Hmm, gif83, is there anything good happening for St Pat's Day? Like, some kind of celebration in Trafalgar Square?