Best neighborhood for several dining options
Depends how much you want to spend on dinner and what kind of food and atmosphere you are looking for but I can give you some ideas.
The three neighborhoods that come to mind are Rittenhouse Square, South St/Bella Vista, and East Passyunk.
Rittenhouse is probably the best neighborhood for walking; it's one of Philly's most bustling neighborhoods and Rittenhouse Square itself is a nice park for a stroll and people watching. Around here, you can get very good cocktails at Ranstead Room (more intimate), Franklin Mortgage (more cramped and loud but has a larger menu of original cocktails), or 1 Tippling Pace (new place, only been once but it was pretty good--they poached the manager from Ranstead and it has a living room type vibe). There are a lot of dinner options, nothing is cheap but there are some good BYOBs that will save you money--Melograno for Italian, Matyson for New American--or some other mid-priced places that I would say are good but not great like The Dandelion, Village Whiskey (many say they have the best burger in the city), Tinto (small plates, can add up) and Pub & Kitchen (a few blocks away but close enough). After dinner you could get gelato from Capogiro or a custard from Shake Shack.
Around South Street, Southwark makes my favorite cocktails in the city. No modern ones here, just classics done perfectly in an intimate atmosphere. The food is good too, especially the housemade charcuterie. For eats, Nomad Pizza a few blocks away has fantastic pizza... a bit pricey for pizza, about $15-20 per smallish pie, but not bad for a dinner and well worth it. It's really good. I haven't been to Chicks across the street from Nomad in a while, and they have since changed the concept a bit, but they always made good cocktails and last time I peeked in it was same bartender I remember. Las Bugambilias is a little closer to Southwark and does very good traditional Mexican in a nice atmosphere. You can also look at Ela for modern New American food but it is pretty pricey. Probably the least nice for walking, South St itself is busy but attracts a young and sometimes a bit rowdy crowd. The walk from Southwark to Nomad is boring and maybe a bit desolate since it's along a side street. If you're willing to walk farther, Zahav is one of the city's best restaurants -- modern Israeli small plates. That's not an exciting walk and it's several blocks that are mostly residential, but you will be going through a very historic neighborhood.
East Passyunk probably has the least good cocktail options, but Stateside does pretty good cocktails and has great food is that is pretty reasonably priced. The bar is tiny so it might be hard to squeeze in. If you like beer, the POPE (Pub on Passyunk East) has one of the city's best beer lists but gets crazy crowded with young hipsters on the weekend. For eats, Le Virtu does great Italian food from Abruzzi, not cheap but reasonable (though a lot of recent posts on this board hint at quality concerns). That's a few blocks away down E Passyunk Ave which has a lot of restaurants and shops. Other good restaurants on that strip are Fond (BYOB) and the new Will (also BYOB, haven't been but initial reviews have been very positive). Again not cheap but you save a bundle on wine.
The problem with the BYOBs of course is that you'll need to tote your wine around beforehand--not a huge deal but may be annoying.
All that said -- getting last minute reservations for most of these places will be tough. If you give more specifics on how much you want to spend and what you like we can probably offer more directed dinner suggestions.