Don't pay for the London Eye or to go up the Eiffel Tower.

Almost all the museums are open some hours when it is free (and London's are free, mainly, all the time, so enjoy them).

Some of the most important artworks in both Paris and London are the public parks. Enjoy them! Eat in them!

In both cities, arab food is usually cheap. And tasty. Street food in Paris is actually very good (crepes, etc).

Gifts? Forget it. They'll still love you. Tell them you thought of them every minute.

Wonderful advice, many thanks!

keep em coming!

Check out these threads:

Beating the £ in London (and the rest of England)
http://www.fodors.com/forums/threadselect.jsp?fid=2&tid=35118693

and

The 100 Best Free Things to Do in Paris
http://www.fodors.com/forums/threadselect.jsp?fid=2&tid=35143882

Don;t waste money on souvenirs for those at home - bring them a postcard or two.

Have amain meal at lunch when the menu of the day is often a avery good deal.

Don;t go near mixed drinks or hard liquor - which are outrageously expensive. Stick to wine, beer or water (refill your bottles from the tap).

A few suggestions:

Almost all the major museums and galleries in London are free--National Gallery, British Museum, National Portrait Gallery, etc.

Viewing the Changing of the Guard is free. Watching the street entertainers at Covent Garden is free although they appreciate a small contribution.

A prettier or as pretty a view of Paris can be had from Sacre Coeur to Montmartre (free) as from the E.T.

Other good free views are top of the Centre Pompidou (more a rooftop view) and from a small self-service cafeteria that's on top of the Printemps department store (no need to purchase anything).

Based on how long you're going to be in London and Paris, figure out ahead of time which metro/bus/underground passes are best value for you.

Gifts--some nice quality food products from Marks and Spencer or similar--jam, tea, biscuits. These can also be picked up at Duty Free.

I think Madame Tussaud's Wax Works is not worth the fairly high admission--others would disagree. IMO, Tower of London is well worth its admission price as is the Louvre (which has some free opening hours).

Sacre Coeur de Montmartre!

For cheap eats in London grab sandwiches in places like EAT, Pret, or even Boots, Sainsburys...and then sit in a park and eat. This will save you paying the VAT of 15% that will be added if you eat in.

Don't buy regular souveniers or gifts for people back home. It's a waste of money.

Instead pick up some small specialty items from a grocery store like fancy candy or cookies in London. In Paris tiny guest-sized soaps in unique French packaging.

Just keep your eyes open for things that are small, easy to pack, not expensive, and buy a number of the same thing. Once in Amsterdam I bought a dozen little colorful ceramic wooden shoes frig magnets to give to friends.

Another recommendation for street food in Paris. My daughter and I ate much more economically in Paris than I would ever have thought possible. And didn't feel in the least deprived! We had a couple of meals at cafes. And a very good lunch at a Jewish restaurant in the Marais (food very similar to the Lebanese restaurants near us here in the Chicago 'burbs; falafal, hummus, etc.). But what we ate the most of were crepes from various stands and sandwiches (with wonderful frites) from a little shop near our hotel.

yk, thank you very much for that link to the thread on beating the Pound! My son and I are in the process of deciding whether we can afford a couple of days in London in June. That thread is full of wonderful info! The search function being what it is here, I'm not sure how long it would've taken me to find it if not for your link!