When outside the battle, you can heal yourself by buying food or drinks from vending machines or cafes. The "normal" world has a lot vending machines, making them a more convenient way of restoring your party's health. Aside from HP and EP recovery, status ailments will be removed and unconscious characters will be awakened as well. Furthermore, if you select a drink or dish that a party member likes, that character will get a temporary LCK stat boost.
You can use consumable items anytime but it is recommended to save these while you're inside a dungeon.. They don't give LCK boosts since they're generic items but they're quite handy for prolonged idolasphere expeditions. You can obtain them as enemy drops, loot from treasure containers, or you can simply buy them from the Heehoo Mart in Shibuya.
There are some dungeons that have healing points. These convenient spots completely heals your party when you interact with them, free of charge. Since you can save anywhere, it's a good idea to farm the area where the healing point is located.
Healing skills like Dia can be used outside of battle but it will still consume EP. Use this only if you don't have enough healing items or if you have more EP recovery items than HP ones. Healing skills are quite potent so use them as well if you need a lot of HP recovered that will consume more healing items otherwise.