The original poster's trip is quickly approaching so I thought I'd post a summary and a few tips for other visitor activites... There's a lot of links here. Some sites are only in Swedish but I've included them anyway in hopes of providing a telephone number or even a picture of a sign to help anyone out.
As has probably been mentioned here, LTHForum in Stockholm would never fly - there are just too few restaurants in Stockholm that are cheap, good and authentic. Many that are expensive and good, though! The exchange rate is the real killer right now - so keep your eyes open and a calculator near your fingertips to guard against some real buyer's regret...
A few, quick general recommendations. You really should make it to the nice market - "
Östermalmshallen". It's on Östermalmstorg ("Östermalm's square") which is on the subway's red line (the station name is: Östermalmstorg). It's worth it for the the oogling alone. I don't eat there (and am actually worried that the restaurants are pushing away the vendors) but, if you've got the cash, you probably can't go wrong in there (I'd personally skip the mezze place in favor of the various scandinavian but that's just me...).
If you don't eat there, do go about two blocks north of the market's main entrance (follow the street "Nybrogatan" north a few blocks) to "
Östermalms korvspecialisten" and order a Thuringer Weisswurst with everything. Take it back to the square and watch Stockholm's uppercrust waddle around for awhile.
Try to order some pan-fried herring somewhere, anywhere. You should be able to find it (look for "stekt strömming") at most all-purpose restaurants. Or, order some at one of the few-remaining herring stands. There's
one at little square ("Södermalmstorg") outside of the "Slussen" subway station. In fact, either have drink at
Gondolen(fancy, order an expensive mixed drink at the bar and enjoy the views over stockholm) or a beer at
Akkurat(non-fancy, incredible beer and whiskey selection) and wander out for some herring afterwards. Both Gondoloen and Akkurat are within a few hundred feet of the stand (in opposite directions of each other, though...).
You'll probably be visiting a museum or two and be sure to check their cafes and restaurants. I've had surprisingly good luck. In fact, I was just at the not-to-miss
Vasa Museum'srestaurant yesterday with my visiting father and my two oldest children and we all ate very well and reasonably priced (for being in what must be Sweden's most popular tourist attraction).
Another lunch and/or dinner tip that won't break the bank is the Opera's "back pocket" ("
Operakällerans backfika"). The main restaurant at the Opera is a one-starred palace of refined food and service (with prices to match) but the "back" or "hip pocket" is a relaxed, informal, no-reservations place serving excellent Swedish dishes. One little tip here is that there is a third option at the Opera house and that's the Opera bar. The menu is the same as the hip pocket but you may (and should) make reservations and the room is small but beautiful.
The original poster's request mentioned that they'd be here over Midsommar. This may certainly effect the opening times of these restaurants so be sure to check! Otherwise, the large, open-air museum/park called "
Skansen" in central Stockholm is your best bet to see traditional Midsommar celebrations. It'll probably be packed so consider yourself warned! Foodwise, there is nearly always someone selling hard-grilled (re: nearly burnt!) herring by the lake in the center of the park. It's a speciality called "sotare" (literally, "chimney-sweeps") and worth finding if you find yourself there.
I'd recommend a trip out to the archipelago sometime during your stay. Check with boats stationed outside the
Grand Hotel and/or
Royal Castleor outside of the Dramatic Theater/along Strandvägen. Another (albeit longer) option is to take a boat to the island community of "Sandhamn" for a day. It's about 3 hours one-way from central Stockholm but many boats serve both food and alcohol and, if the weather is nice, the time could fly by. The community itself is quaint and charming and the best lunch bet is at
Sandhamns Värdshus. Yet another boat option is a shrimp (all-you-can-eat shrimp) or jazz cruise on one of the
100 year-old steam engine boats.
Finally, if you want to bust the bank, Stockholm does have some great high-end dining. I've been to both
Restaurang GQ and
Leijontornet relatively recently and enjoyed every bite/sip. Leijontornet is especially doing some cutting-edge stuff with many local/Swedish ingredients and inspirations. There are probably about 5 other restaurants in this category worth mentioning (Mistral,
Lux,
F12,
P&N,
Edsbacka krog,
Mattias Dahlgren) but I haven't tried them. FYI - tasting menus will probably set you back about 150-175 bucks/person without drinks or tips (modest but expected at this price class).
Don't shy away from going to a market or food specialist store and picking up some breads, fruit, sausages and cheeses and making an informal picnic somewhere, practically anywhere. Keep your eyes open for the government-owned liquor stores,
Systembolaget, if you want spirits, wine or beer with alcohol content over 3.5%. Stockholm does have open liquor laws but keep things reasonable and under control and I'd be shocked if you were hassled in the middle of the summertime.
That's about what I've got but Stockholm has much to offer. Please let me know if you've got any special requests!