I'd like to add on to some of this.
Loki: Fire. He is a fire-etin, just as Surtr is. It is, therefore, appropriate if you desire to burn offerings for him. I like to craft figurines out of wood and throw them into a ritual fire for this purpose. He also like things that make explosions (from UPG*) therefore fireworks are said to be good. Choclates and sweet things are given to him as well. It is also traditional to pour a drink for him when one pours one for Odin. Playful pranks sometimes appreciated.
Surtr and Sinmora: Fire, fire, fire. Anything with fire. Burn it. I tend to take food offerings and burn them in the ritual fire. These two, being strong fire-etins, are also very appreciative of joyous music and festive dancing. Dancing around a ritual fire is a good type of offering. Sweat as well, since sweat comes from heat. They also approve of hard work and dedication.
Fenrir: Bloodied meats (raw if you can), animal skins (from things you have hunted), bits of bone, blood. Acts of strength. Emotions, such as anger. He doesn't tend to be too big on offerings though.
Hela: Blood, bits of bone, decaying foods (if you don't mind letting things rot), blackened roses, bitter drinks and hardened breads.
Angrboda: Feats of physical and mental strength, fortitude and effort.She is more a deity who appreciates abstract offerings rather than physical ones. Dedication and oath-to work is a good kind of offering for her. She appreciates abnormalities and ovecoming them. If you do give her food, I would give hearty drinnks and dark meats.
Nidhogg: Anything decaying and wasting away. I set up a garbage bin for Nidhogg, and a compost pile. Throwing away offerings into Nidhogg's compost is a good way to get rid of things you've let rot and also still give to Nidhogg.
Mimir: Bits and pieces of knowledge, a great riddle or puzzle, and much booze.
(*Note: A lot of this is UPG/PCPG).