Hi M.,
Ideas.... I would invite them both for Sunday Brunch at Babayegas (if your mother can come back during a weekend) The Brunch there has a very reasonable price and it's excellent $17.95 per person.
Idea No. 2 Manicure and/or pedicure at their preferred location.
Idea 3. Easter is coming, you can get at Marshalls a very reasonable priced Easter Bunny, wreath or anything decorating. (You mentioned they watch what they eat so I am not suggesting chocolates, cookies or anything like that)
A lot of times we like the little things like soaps or special body creams but we never get to buy them for ourselves...you can also find something nice with a great smell at Marshalls.
Personally I like the brunch idea, that is if your Mom can make it. http://www.babayega.com/brunch.html
Good luck, I will love to hear what you did!
M.