Hello - Nova Scotian here - 10 days is def enough. Most people only give NS 3-4 days - 10 would be amazing for NS and PEI.
PEI is fairly small - I mean you can drive one end to the other in 3-4 hrs. A few nights there would suffice. You can look at taking the ferry from Pictou,. NS, then come back across the bridge (the bridge is cheaper than the ferry and you only pay when leaving the island). I will say - it's been...um...I think 2006 since I've been to the island. Charlottetown is a nice visit, we enjoyed Summerside, if you like Anne of Green Gables be sure to visit the house. Cavendish (last time we visited) was a bit touristy - with a wax museum, Ripley's Believe it or Not - nice beach tho. We enjoyed our visit to New Glasgow (not the one in NS) and the butterfly house/gardens. Woodleigh Replicas - well, last time we went, it was getting unkempt. Two nights, or 3 if you want to meander.
A few days for Halifax is sufficient - the public gardens, walk the waterfront (which is a bit under construction with the Queen's Marque), Maritime Museum of the Atlantic, Citadel Hill, Natural History Museum, make sure to visit the public library on Spring Garden Road (it won some recognition for it's architecture and is modern and lovely). I mean - all that can be done in a well planned day, and all these areas are walkable from each other. Head out for a harbour cruise - the Mar II or the Harbour Hopper.
Peggy's Cove will be crawling with people - if you can get there early or after 5pm or so, it'll have fewer people. PLEASE - stay off the black rocks - waves can come up and we've had a few people swept off and drowned in the last few years. Take a stop at the Swiss Air memorial in memory of the plane crash. With Peggy's Cove, you can stop into Chester (never been, embarrassed to say) and Lunenburg - UNESCO world heritage site. We also go to Mahone Bay a few times a year, but they have some great festivals and my husband's cousin lives there. Usually we just shop. All this could be done in a well planned day (you'll have lots of daylight hours in June). And Oak Island is just a short ride to Mahone Bay - you could also look into The Ovens Natural Park (We haven't been there for decades - hubby says it was falling apart).
Depending on what you want to do in Cape Breton, you could stay 1 or 2 nights. I'd say 2 - especially if you want to do some hiking or visit Fortress Louisbourg.
Haven't been Digby way for ages. Annapolis Royal might be a fun stop for you. Wolfville is a great university town. Make sure to go to The Lookoff for lovely views over the Valley. There is some good whale watching as well, Digby way. I hear Kejimkujik (AKA Keji) National park is nice, but I've never been. Oaklawn Farm Zoo is a nice stop. This could all be done in a day.
If you like getting wet, you could look into Tidal Bore Rafting. Burntcoat Head park is nice - check for low tide hours and you can walk on the ocean floor! (Alas, we went during high tide). Truro (my town) has a nice park with some great non-strenuous hikes...Victoria Park. If you were in Truro on a Sat morning, our farmer's market is nice as well. Make sure to stop into Masstown Market for an amazing bakery and seafood. Shubenacadie Wildlife Park has lots of animals as well (nothing exotic - mostly Canadian animals that were rehabilitated but can't be released).
Parrsboro/Joggins and the fossil cliffs could be a nice visit as well if that kinda thing floats your boat.
So - Halifax - one very full day (or split to 2). A day for the South Shore/Peggy's Cove. A day for Annapolis Valley/Digby. Toss in a day if you want to visit Keji. Two nights for Cape Breton.
And I'm missing a lot of places, so you can certainly toss in some others! Antigonish. Pictou. Sherbrooke Village. Tatamagouche has a great walking trail and a village train.