I download the apk and obb from last soft release and here's what I have found that's new in the Summer Parties update:
1) Strawburst released.
2) Cactus avaible (soon to be released)
3) New costumes for some plants: Strawburst, Cactus, Lightning Reed, Peashooter, Stallia, Gold Leaf, Tall-Nut.
4) New quests in Travel Log: kill X amount of Gargantuars from FF, WW and AE; kill X amount of Wizards; kill X amount of Seagull Zombies.
5) New summer piñatas parties.
6) NOT sure if Frostbite Caves' sliders were fixed but Travel Log's bug (when a quest don't disappear even after you completed it) wasn't fixed.