Photographical topics in IR
One of the most important characteristics of IR light is that all green (and coloured, such as flowers) parts of plants have a very high luminosity. Tree stems are not bright. Photos of trees show a strong contrast between the stems and the leafs. Images of foliage combined with sky, water or stone have an awkward and very interesting contrast because the foliage is much brighter than het dark sky or buildings.
A good example of this effect is climbing plants on ancient buildings. Foliage shows very bright against a dark stone background resulting in a beautifully strange image. Another possibility is gravestones in a lawn. The strange contrast increases the spooky feel of the graveyard pictures. A downside is that pictures combining flowers and foliage may not be very interesting because the flowers will show the same as leafs.
A good portion of blue sky (possibly with some nice clouds as accent) really shows the Infrared character of IR pictures. Blue skies are dark, and often have a nice graduation with the lightest part near the horizon. The dark sky has a nice contrast to most other objects in the photograph and ads some to the tranquillity of the otherwise often quite busy pictures. The effect of dark skies is strongest when photographing with the sun behind you. When the sun is in or just outside the frame, the sky will show lighter and you risk lens flares which are often more pronounced in IR then in visible light.
Technique \ architecture
Photos of technical subject or architecture are often aimed at the shapes and these photos are nice in monochrome (toned or true black & white). In IR photography the contrast between illuminated and shadowed shows stronger than in visible light. This effect can be used to accentuate the shapes even stronger. Old buildings, modern architecture, trains, cars and boats are rewarding subject in infrared photography '
Photographing people in IR is very well possible, but not everybody likes the effect. Skin shows very light, almost translucent and discolorations or impurities do not show. The eyes (especially the pupils) contrast with the skin and show very dark. Dark hair shows dark and light beards may show quite strong. In clothing the contrast often change a bit because many pigments have a different effect in IR. A striking feature is that some black clothing shows rather light while other black are truly black also in IR.